This is a spin-off of the thread about havens.
Extraterritoriality: what is it?
According to canon materials, it means that certain individuals are immune to national jurisdiction, as opposed to the jurisdiction of their sponsoring corporation. It also means that the marked, differentiable property of a corporation (of at least AA status) is not in the jurisdiction of the nation including it.
In the real world, historically, there are two basic models that resemble this. The first is diplomatic immunity, which is a special case that clearly does not apply here (there's no supply of credentials, no revocation procedure or anything similar) and the second case relates to the demands of international corporations (and their sponsoring governments) in the imperial age. At the time they were seen (as in the case of China and various foreign governments) as a national humiliation, and wars erupted because of them (such as the Opium Wars).
This second case is a lot closer to what is being presented in canon.
So the question this raises is: why would nations worldwide submit to such a humiliation? Especially since according to canon, quite a few simply ignore it, without apparent catastrophes following. Both Tirs, as well as Manchuria are on that list. As presented, none of this makes a whole lot of sense. I could see a few really broken or small jurisdictions being bullied into accepting the state of affairs, but I can't really see a reconsolidated UCAS, CAS and others accepting it without someone holding a gun to their heads.
So what is more plausible?
Let's start by understanding the scope of the result, the obstacles, and what similar cases exist today.
What we are talking about is a removal of power and autonomy from governments. Governments are not in the business of giving those up, and in fact regularly expand their power in the course of doing business. The exceptions come about when the governments are weak for some reason, forced by war or popular protests or enticed by greater gains elsewhere.
An example of greater gains: trade treaties, in which there are stipulated dispute resolution mechanisms (for instance NAFTA). Governments want to promote wealth and productivity to make their populations happier, and in this cause they will establish a process for differential treatment of corporations.
If we look at canon extraterritoriality, there's no balancing gain for national governments. More money? Nope - in fact they arguably lose sources of revenue. More power? Nope - they give up jurisdiction. More glory? Nope - it's humiliation all around.
What would it take to make a government accept a massive humiliation? If history is any guide, overwhelming force. Period. Do the megas have massive military power? No, or at least nothing to compare with national forces, and they sure as hell don't want to look imperial if they don't have to.
So there has to be a quid pro quo. There has to be some reason for a government to offer up privileges to big corporations.
Here's a thought.
The first model is that a corp comes to a government and says: "We want to establish a company town, under our own rules and jurisdictions, and we want our executives to have special legal privileges that place them beyond the power of your regular cops. But wait, before you laugh and kick us out, this is going to be so profitable for us that we will pay an enhanced lease fee for the land, which will more than make up for tax losses, and you get to save money on policing, and of course government officials will get cushy jobs with us on retirement."
This works, because it can be justified on the grounds of cost, of profit, and of corruption. It also works because the same corp can, five years later, say: "Y'know, we'd love to expand our holdings here and pay you many more times the cash while we establish huge factories and employ thousands of people, but La Republica de las Bananas is offering us a much lower lease rate. Would you like to sweeten the pot with an extradition agreement so that miscreants can't escape our corporate justice?"
Pretty soon you may have a few megacorps arranging similar deals on similar terms, and now instead of shoving something down the throat of a government that can simply revoke a corporate charter, pierce the corporate veil and shove everybody in jail for criminal conspiracy and pocket the cash, you have a clear quid pro quo arrangement.
It then only makes sense to have multilateral talks between megas and governments in which the terms of these deals are regularised, with governments giving up limited chunks of jurisdiction in return for fat sacks of cash money, plus wonderful prizes for their servants.
Another model might be where you have a truly wrecked piece of territory. It's rife with guerillas, ghouls, or just plain toxic waste, and the government says to the corps: "My friends, we're setting up an auction. Whoever will clean up this place, may within ten calendar years arrange a long term lease (999 years) on this property and hold effective independent jurisdiction over this area. Here are the documents laying out the area defined, the terms of approval for what constitutes adequate rehabilitation of the area, and you can submit your bids for the lease terms to our office here. Don't be strangers, now! Er, the reserve price is $100/acre/year in 2020 prices."
In this case, the government willingly opens negotiations for very clear reasons, with clear benefits for the corps as well as the government. The public interest is nominally served, by cleaning up blight, and it offers corps a foothold in a world gone mad.
Again, to rationalise the different cases, a wide-ranging treaty can be negotiated some time after the fact.
I'm sure that there are other models, and I'd love to hear other people's ideas.