Are roll bars legal for street use?

There's nothing illegal about a roll bar and harnesses.
When you report those modifications to your insurance company (as you must legally do) make sure you're sitting down when they give you the new price for your insurance.
Track style modifications make an insurance company think "Whoa, boy racer !!".

Roll bars and cages are legal, but in most states, a racing seat belt harness is not. (I know they hold you better and are actually safer in a crash, but I'm not the law makers!) I had to have two sets of belts in my street/race course car. The standard shoulder belt/lap belt system (for legal street use) and the 5 point racing harness.(S.C.C.A. racing requirement) On the street, the racing harness was tucked out of the way and on the track, the shoulder harness was buckled behind the seat.

The law doesn't allow them because it is harder for emergency personnel to release you from a racing harness if you are unable to help and thus in case of a fire (extremely rare occurrence!!!) the emergency personnel might not be able to free you.

BTW: If you call a roll bar a "Roll over protection bar", the insurance on my convertible sports car actually went down. (less chance of serious injury during a roll over versus no bar, where the person literally loses his/her head in a roll over.)

Probably it is legal, but (as a friend found out) insurance goes through the roof.

Yes