1. They don't want underage nudes on their servers, even if they are exchanged between consenting 17 year olds.

2. They got feedback from some focus group who said the worst part about other online dating apps is being given unsolicited pics or being asked for more pics.

This "casual encounter" bit is their chosen marketing-approved euphemism to explain this, or maybe it tested the best. I honestly don't think they care how this gets used, but I'm sure they tested all kinds of language and this did the best.