Since we're probably dealing with a Pirates of Drinax campaign in the OP, the laws and regulations that apply within the Imperium won't count for much. Nor are all of those suits going to be stolen. Given the history of the Reach (especially that of Sindal and its successor states) there'll be plenty of suits floating around that were duly inherited, salvaged, and/or manufactured there, which could easily include psionic variants, and they're apparently accessible enough that Prince Harrick can end up giving one to a PC in return for trying to secure him a party invitation. (Granted, it's not just any old party, but still.)
That doesn't mean that you'll have dozens readily available on the open market, but if you're making deals on Theev, a suit or two is certainly possible. (Even a new one: GeDeCo and other manufacturers operating there don't seem to mind selling outright WMDs in Blacksand, so I doubt that they'd develop a sudden case of scruples about powered armor.)
As for wearing it... well, use a little bit of sense. Wearing it during combat operations is reasonable, especially for a pirate. Taking a casual stroll through town wearing it will draw unwanted attention, unless it's the sort of place where the only law that exists is at gunpoint. (Okay, if it's the suit that Harrick gifted you then it's probably also ceremonial and history-laden enough that you could wear it around the palace... just like a bunch of the Star Guard do, so mind your manners.)