Almost all restaurants have a disclaimer that parties of a certain size will be assessed a gratuity.
You get around this by asking for separate checks for each couple.
Doesn't work over here. Every now and then me and a group of about eight to 10 people will pick a restaurant or bar and enjoy ourselves but we all start our own tabs. Some of the restaurants know we're together and charge gratuity anyway. When they do that, I don't leave a penny more than I'm charged, but my service was always good (I won't say exceptional), so I never argued it.
Typically in restaurants
I tip about 10 percent, though at bars I tip about 20 since they're also mixing the drink, and on two occasions I dropped a 50 percent tip, though one was for playing along with me on a joke with my brother... If they give me shit or if they're ignoring me, I limit it to a dollar, and if they really piss me off, I leave only a penny, and I place it in a prominent place on the table so the waiter/waitress knows I didn't forget to tip, but that I just thought s/he ruined my night.
As for the restaurant, calling the police for arguing the gratuity is the dumbest PR move they could have ever done. I hope the restaurant goes under.