It's really the principle of the thing.
I don't think anyone with functioning brain cells needs a tip ESTIMATOR. We can already do that; "Let's see, bill is 48 bucks, 10% of that is obviously 5ish, twenty percent would be $10ish."
Gee, I don't need an app for that.
That said, it's the developer's app, and he can have whatever features he wants.