OK, this is crap.
1. You calculate tip on what you bought, not the tax. You can pay whatever the hell you want, but you are just being a little more generous on the after tax.
2. Adding tax to the amount separately is a sham. They should just include it in the menu price. You say $15, let me PAY $15. Nevermind $16.05 because of tax (this is really a bother at a bar/grill. I pay $5 for a drink at the bar, but $5.35 at the table? Add to this the typical buck tip on a brew vs 15%. I would pay $6.10 at the table and the waiter would only get $0.75 or I pay $6 at the bar and give the bartender $1. Absolute BS)
3. there was a legal case on the group tip thing. If the writing calls it "gratuity" you do not have to pay it. If they call it "service charge" you do. Legal lingo, but one is still defined as a "thank you" and the other as a billable service/charge. Convincing the place you are at may be something different though.
Blame #3 on the asshats that will go in as a party, leave a huge mess and leave crap for tips because $20 LOOKS like enough, but is beans for a meal that summed up over $200.....