That sounds true in principle, but your ignoring the commision aspect of the service. In the tipping model, a very efficient server could, say, serve 100 miffins/hr and be tipped 20% each accordingly. A fast, but rude server could serve 100 muffins, but perhaps get 0-10% tip each sale. A slow, but polite server may only serve 50 muffins, but garner 20% tip each.
If you simply jack the muffin price up and distribute equally among all servers, as you suggest, then there is no incentive to perform above average - no lesson learned for lackluster performance.
Personally, I tip anywhere between 15 - 30%, understanding that this is part of the servers anticipated income. If service is bad enough for me to want to tip 0%, then I either leave before I place an order, or I tip 15% then don't go back then write a negative Yelp review to warn others.