In America. This would not fly in Europe, I hope. Pretty sure menus have to be displayed out front with full pricing and service charges have their own rules.
I'm sure that if one local restaurant implemented this they wouldn't last long, but if a lot of local restaurants started doing it at the same time, they'd probably get away with it. Maybe they're all waiting for someone else to take the plunge.
Many other places add it routinely and have it listed in their menus. Not just for parties of 6 or more, but two guests.
America is a big place, so is Europe. I wouldn't expect this to fly in a lot of places in America either. I've seen a place in America where they tried it and it failed.
Online, you can get away with (figurative) murder.
Saying that I have started seeing gratuity automatically applied to some of my bills... So perhaps it's not as illegal as I thought...