Jerry has it about right. Greg is describing the peculiar UK irony that if you see a folk club advertised you usually expect unamplified music, but when you see an "acoustic music" night you almost know there will be a PA system.

It's illogical, but I think it reflects a different approach - what the "acoustic music" people mean is that they are using acoustic guitars (yeah, instead of a normal guitar...?) and more generally that no electric instruments as such are used - which is different from acoustic instruments + sound reinforcement. If you come from a background of rock'n'pop (which I don't), I suppose you need an adjective to make that distinction.

Anahata