On Saturday in Leeds I stopped to listen to a busker playing classical music on a battered old guitar through a small amplifier which he was sitting on. He wasn't over-loud compared to any of the other buskers around and I suspect he would have been drowned out by the street noise of a busy city centre.
I think it's a case of using common sense and showing a little respect to others, when using amplification in public areas.
The demonstrators protesting about the search for weapons of mass destruction in Iraq, while entitled to voice their opinion, were rather heavy handed in their use of a megaphone to make their point.
It would have been quite impossible to busk anywhere near them with or without an amp.