To be a little more specific, there is no risk when a group of friends sing happy birthday to someone they know in a restaurant. However, if a restaurant's staf

To be a little more specific, there is no risk when a group of friends sing happy birthday to someone they know in a restaurant. However, if a restaurant's staff routinely sings the song to customers, then people will come to that restaurant specifically to receive this service - which is profiting off the song.