I don't think that anyone minds removing copyright notice as such, but some sites like to integrate BB's completely, so it looks like they have developed custom solution themselves. In order to maintain consistant look, they only want one copyright notice (their own) and not two.
Now that I am aware of YaBB existance I will test it on my dedicated server. The fact that I can pay and remove copyright notice just might win the day for YaBB, as 'the vBother' forum software would not allow me to do that.
