On the other hand I think you can post your donation link in your signature or in your personal message space like what I did.
In signature is passive though it appears with other posts. If you allow donations to be asked for via posts we would have TONS of threads of utter crap asking for BTC. Sadly most of this would be fake stories, there might be a few real ones but would be so abused. Some forum topics like beginner I think would be horrible if they allowed it.
So allowing in signature is a way to let people put it and not have a forum full of posts. They most likely will get no donations so it is not really a big thing.