I think its all nationalism
I would argue the opposite, even before his last fight Bisping would get massive amounts of hate from Americans and Canadians. I attribute a lot of this to the TUF season against Hendo. Here is is immediately cast as the bad guy by virtue of not being the American in a Nation v Nation season. From what I remember, Henderson was a bigger d**che than Bisping in that season. The fact that Henderson (apart from already having garnered a respectably fan base) represented "team USA" immediately makes him a "good guy" to the casual (and even hardcore) American fan. Therefore Bisping immediately becomes the "bad guy", a feeling which seems to have stuck. Just like Rashad became the "bad guy" in his season v Rampage even though I felt he was the better coach and more mature figure during the show.
I am not saying he (Bisping) is necessarily a good guy, just not as bad a guy as many people seem to believe. He has had negatives, primarily the Hamill fight, his reactions afterwards and his actions in his last fight. I have heard condemnation and excuses for these two major "scandals" in his career and personally fall in the middle and might even sway a bit towards the Bisping side on both counts. Not in condoning his actions but being able to understand them.
If it wasn't for his statements after the Hamill fight, people wouldn't mind as much. Sure he was gifted a decision which he shouldn't have gotten, but usually this only results in the loser of a BS decision gaining in popularity and not the winner being lambasted. e.g. Leonard Garcia isn't hated for the judges being idiots, but now everyone loves Nam Phan. And Garcia even went on record afterwards saying he deserved to win the fight. Similarly, while this fight made me more sympathetic to Hamill, it did not cause me to hate Bisping.
I have heard that in the lead up to the Hamill fight he lost his father, which would f*ck up anyone. In the Rivera case (aside from the blatantly illegal knee) I can understand his actions as well. One memorable post I read said something along the lines as "spitting is despicable and would result in a fight where I come from". People seem to gloss over the fact that Rivera got intentionally and unnecessarily personal implying Bisping's girlfriend has a StD. Where I come from that would be much more serious than spitting. Especially since he spat at someone who had been talking smack at him, yet couldn't get in the ring with him. So far in watching UFC events cornermen and trainers usually are very respectful, talking up their fighters but not dissing opponents. That just seems incredibly unprofessional.
However Bisping apologized (repeatedly) and while he may just seem to be trying to save his own ass and towing the company line (as he seems to be in my opinion), I haven't read a team Rivera apology anywhere even though they acted terribly unprofessionally as well. In fact he seems to have gained fans for his actions. Personally for me neither fighter came out looking good from that debacle and while I am disappointed at Bisping for his actions I am equally disappointed at Rivera, I am even disappointed at the many people who lionize Bisping while refusing to hold Rivera to similar standards.
I realize this is an incredibly long post and my opinion can be summed up as "meh, he is a dick but sometimes has reason for being one", but the OP asked for an opinion actually supported by some arguments and that is what I attempted and I'm not even a staunch Bisping supporter, I just feel he is treated a little unfairly by most fans. I'm trying to look at this from as unbiased perspective as possible. While I realize one can never be completely unbiased I like to believe I fall stronly on either side of the debate. IMO I see Bisping as I see any other MMA fighter, I will either root for or against him on a case by case basis.