It's not hard to understand this hateful attitude towards prot warriors. Last year, someone was curious to know why brown tanks get mistreated by the playerbase, and some posters explained the reasons in full detail. You can check their reponses here:
http://forum.warmane.com/showthread.php?t=428444
Honestly, back on Frostmourne I've played alongside a very skillful prot warrior, who knew boss tactics and how to play his class well. In the following video, you can see how another brown player tanked LoD:
So, admittedly, playing as a brown tank doesn't mean that the person is bad in the videio game. But in some cases, such comps might not be optimal and could require extra attention from the healers, in order to ensure the tank's safety. People don't want to risk with bunch of strangers or be burdened during boss encounters, and thus they prefer their progression raids to go as smooth as possible. This reinforces the hate towards brown tanks. It's unfortunate, but that's a deisgn flaw by Blizzard devs coupled with the min-max culture - they gave some classes more benefits than others, which led to more narrow comps. This isn't present only on wotlk. In Vanilla/Classic, warriors were/are the best in dps, while druids were/are the laughing stock.