It's irrelevant whether you can skin other people's mobs or not. The fact of the matter is, if you *want* to gain experience while skinning, you *can*. You aren't forced to take the option of killing to skin, but you can do so if you choose, so if you choose not to, that's entirely in your hands.

Miners and herbalists have no real options. Sure, there are a few mobs that drop herbs and ores, but very rarely, and they don't improve your skill, they just give you the mat. So they have to mine and pick herbs if they want to skill up, and unless they get XP while doing so, they're not advancing their character's level during the time they're "wasting" working on their profession. Fairness to them means allowing them to gain XP while mining and picking herbs, just as it means allowing you to gain XP while killing mobs for skinning. It's still slightly unfair in that you can choose not do (by just skinning other people's kills) whereas they don't have that choice, but it's more fair than the alternative.