Both sides are at fault. The loudest haters commonly go with "cancer" and "fags". But people attacking MLP are directly attacked themselves by the loudest bronies. It's a common cirle.
If you bring out the argument that MLP is always attacked and being called cancer. People will say it's everywhere. They don't like to go to places just to see it filled with something they dislike (MLP in this case). You can't blame them, bronies do sometimes start forcing.
Both sides overexagerate at the other side. It's how the internet works. But the loudest talkers are usually the same type of people. If 10 people see a pony and 1 person goes "CANCER!" it immediatly seems like an attack. But it's the minority that does it. Most people prever to stay neutral, but we never see those, because they stay neutral.
The argument of "bronies immediatly thumb down negative comments" is also a common one. But that's not 100% true. Attacking comments get thumbed down, but that makes sense. Calm negative comments, not directly attacking, stay alive. Attacking haters happens on every page, everywhere. Try saying something negative about Pokemon on a Pokemon page. But because the bronies visit more often and are larger in number, it just seems to go faster.
So haters and bronies are actually just the same. The fandoms are just different.
I prever to stay neutral and ignore the attacks from both sides. I like MLP, just let me have fun with it. I won't annoy others who dislikes MLP with ponies.