Is it legal for a shop to charge a difference price for foreginers?

One of my friends went to a Big Echo karaoke and was forced to pay a higher price from a "gaijin menu". He speaks Japanese so he could understand the situation, but couldn't negociate out of being charge a premium…

Is it legal to do this here?
I've lived in Tokyo for 12 years and it's the first time that I hear about this kind of separate menu.

by emmanuelgendre