Do you think the sisters made it clear before ordering that they would require separate checks? If not, no wonder there was a long wait. Think what the servers had to do to create accurate checks for each person.
At many restaurants, large parties have an automatic 18% gratuity added.
I wonder if the patrons were trying to duck out of that.
Why the auto-tip?
Because servers are taxed based on the bill.
Not starting a tipping debate here, it's just the way the tax laws work.