Also there is nothing that FORCES you to pay to the other person, no legaly binding contract so if you really don't feel like you're just gonna say "fuck it and fuck you" and it's gone.
Your second point shouldn't be an issue for anyone who has some level of maturity and/or values their relationship with their friend... not like it is likely to break a friendship but it seems like a dick thing to do.
You're correct. It's optional. There's no contract. But that's exactly the point.
It's a simple nudge. I tiny but effective behavioural hack.
The alternative is what? The status quo (which isn't working)? I guess the question is: what's your answer then? That's not being troll-y. I'm seriously curious about other effective alternatives.