Sez who?
Delta Grade 'ware is no more or less illegal than any other grade... sure, if you go for wired 3 or spurs or anything else with no permit available then its Illegal, but it would be anyway.
As payment for a special run once my character recieved 5 Million Nu
of Deltaware, which i used to delta all my existing stuff and put in a new Delta VCR-2. As all my stuff was Wired 1, skillwires-4, headware radio, telephone, datajack, smartlink, etc etc all of which was legal or has permits then my delta'd character has no appreciable legality consequences of being delta'd.
Now obviously if i had chosen to get Wired-3 as an upgrade I'd have legal issues, but no more than if i'd had everything ripped out and Wired-3 put in at standard grade.
Now ok, my character had started life as a Covert Ops/Decker type, but was now going to be a Rigger (mainly), rather than being a Sam looking to max combat power, but it still doesn't mean that typical deltaware is 'usually military grade illegal stuff'. It only is if you choose to go that route.
There is an awful lot of legal or permit cyberware that can have very useful essence savings as delta grade.