Unfortunately, the above posters nailed it on the head.
There has been massive abuse of the BH cap ships in the past, and it continues in many forms today. The reasoning behind the whole bounty restriction, as well as the lock on unauthorized BH caps in house space is because there have been many, MANY incidents where Hunters would simply camp unlawful stations or charge into roleplayed fights and blast at everything red.
As a Junker I have to say that Yanagi is ground zero for this nonsense, it being a central launch point for pirates and being in scanner range of the most abused trade route for oorp powertraders. I don't know how often I've undocked only to get blasted by two BCs without a word of roleplay, or anything besides 'ur ded leave sys now'.
It still continues today, as I said. Just yesterday the GMG and myself (as a Junker) had an incident with a certain import faction that flits about in Bounty Hunter Battlecruisers. They interrupted roleplay in progress to shoot at a solitary unbountied Outcast gunboat, browbeat the GMG for not killing every Corsair and Outcast in sight, and then got into a firefight with said GMG pilots after a heated argument broke out.
It's unfortunate as no other faction takes so much crap as Bounty Hunters, but no other faction has had so many who just abuse the tag as an excuse to go blow up every red-HUD player in the biggest ship they're willing to powertrade for.