Yeah, by law, anything that's not DOT approved & should be (like lights, tires, etc) isn't legal for road use.
And I don't know of a single aftermarket HID kit that's dot approved... way too much red tape & money to get that, especially without replacing the housing, since the reflector was built for a different bulb.... Plus they're just way too super awesomely baddd azzzz to be confined to the socialist communistic ways of the LAW!
But like you said, so many cars come with them stock nowadays, if you go with a stockish color there's very little chance of you getting yanked for them... but if they're super purple, well... you're still not guaranteed to get pulled over, but the risk is certainly higher depending on how anal the cop is that you just rode past.