Defaults matter. If the 'default' tip on the app is 15% or 20%, then you need to make an affirmative choice to not tip. Yes, you can (usually) opt out, but the screen is telling you in no uncertain terms that tipping is probably a social norm here—if you choose to opt out, you are making the extra effort to break the norm . Otherwise, why bother setting the default at all rather than force the user to enter an explic…
I like when they set the defaults even higher than 20%. I went to a Sports Clips and the lowest default was $5 on an $18 haircut, with I think $10 being the highest. They spend probably 20 minutes on a basic haircut. I don't think that a 50%+ tip is reasonable.
Then I learned that it's pretty much expected, so I tip "normally" now, but feel weird about it.