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 explicit amount every time, like on a restaurant bill?
Also, there's a whole subbranch of behavioral economics ('nudge' economics) that basically studies how to frame choices and set defaults to influence decision making [1]. Advertisers and politicians also take advantage of the power of framing and defaults.
So getting back to the ethics of the issue, it'd be interesting to see data on how much the default influences the actual tip paid, and how that incentivizes the actual business owner (e.g. set a higher tip rate, pay a lower wage?)
[1] http://faculty.chicagobooth.edu/richard.thaler/research/pdf/...