They have other games (STO off the top of my head) giving them examples of what they can get away with doing to their player base. Heck, TSW charges you $10US to buy a new outfit for your pretend person and Marvel Universe (Open beta weekend starts today if anyone wanted to play) charges you $10 per costume.
When one game gets away with doing things like that, and they don't see it negatively affect their numbers, well they all think they can get away with it.
So before you say "I have no problem paying to" just remember that the thing for which you have no problem paying may only be the thin end of the wedge. When the companies see how many people have no problems paying for X, well they may as well just charge you for W, Y and Z while they're at it.