I don't know about in restaurants, but in many shops music is used as a marketing tool according to an article I read a few years ago.....sorry I can't link to it, can't find it, but it was in the Toronto Star. Shops pitch their stock and music at a certain demographic, and if you don't like the music played in that shop then you are not the customer they are after, it seems.

A local cafe to which Himself and I often go for a coffee has soft music playing, often gentle classical....not Wagner.....at just enough volume to be heard. It seems to make kitchen noises recede into the background and give privacy to other people's conversations, without being intrusive itself.