There's a logic. It reinforces labor aristocracy but it's not like McDonald's is busing a table, giving you a place setting, or an employee whose narrow responsibility is making sure you get what you want.
Sweating percentages on small dollar transactions is a waste of your time. The $2 between tipping ten or fifteen percent on that $40 TGI Fridays tab might have a great deal of marginal utility to my server but I couldn't care less and if you do you should be going on dates to Taco Bell instead.