Ita.I can't back this sort of petition because these people are harmlessly enjoying Disneyland. The "Show Your Disney Site" promotion itself showed a stereotypically biker-esque person enjoying the park. These people know that their outfits create an ironic juxtaposition, and it is harmless.
That is a far cry from actually promoting a real-world gang at Disneyland, and that difference is the reason one isn't permitted and the other is. I have also come to note that the idea that they are ruder than other guests, or cause issues is basically an unfounded lie to try and justify banning this action.