The account of the exchange is unclear, but here are some simple questions that I hope are provocative.
What makes something comedic? What is it that a comedian is doing? Are you a comedian if you make someone laugh, or do you have to be on stage? If you're on stage, do you have to tell jokes that would make people laugh in a more private setting, or can you let the performance take over? Are you a comedian, telling comedy, the whole time you're on stage? Is it enough simply to intend to make others laugh, even if you've missed the mark? And in what circumstances can a comedian be assessed as anything other than a comedian, a clown, a joke maker?
My opinion is that comedy does not take place on a stage, it is not a sustained act that begins when you take the mic and ends when you drop it (oh snap!). When you stand on stage, much like when you talk with friends, you take part in the highest art of comedy for moments, hoping to punctuate your commentary with the smiles and laughter of others. Although history is replete with "the comedian's comedian", the audience needs to feel like they can find the subjects of your comedy entertaining, whether it is normally a serious conversation in other social contexts or not.
It is for precisely this reason that I think Tosh made a mistake. First, because you're playing to a social group (which is usually quite variable, but is increasingly homogenized via globalization), you need to take into account what that group might think is funny. If follows from this that it is possible that some things are easier to joke about at a time in the historical past (think about, say, the subservience of blacks) than it is to joke about now, and I don't think there's anything really wrong with that. It also follows that when the audience has explicity said "I don't find that funny", and when the general social norm tends to be that it's not a topic with a lot of humour surrounding it, you might want to steer clear.
I do agree with Tosh that some things which are normally difficult or offensive to talk about can be discussed with humour, sometimes even better than they would have been discussed otherwise. However, standing on stage with a mic in your hand does not give you license to "joke" about how "funny" (on some views, that might be synonymous with "awesome") it would be for someone sensitive to rape to get gang-raped. If you say something like that without being absolutely clear that you're talking about meta-issues (as Tosh claims he was trying to do), then you should completely expect to get rebuffed. Rebuffed with extreme prejudice.
Also, I love Bill Hicks, but he is clearly a totally different comedian than Tosh, and so the comparisons are a little weak (I take it that a comedian known for dabbling in ribaldry has more leeway with their audience). Even then, the fact that someone else did something is no reason why someone else should get to do it, too; you wouldn't let your kids try such a silly argument, would you?