Overall I think this is a great move, and it's good for founders going through the batch. But they could have reasons to not want to give more equity to YC (maybe have more room in SAFEs for strategic angels, stuff like that).
edit: I originally called 'more equity' pro rata, which is not correct at all.
> a pro rata clause in an investment agreement gives the investor a right (but not the obligation) to participate in one or more future financing rounds to maintain their percentage stake in the company.
This deal explicitly says "hey, here's 375k; we'll take whatever share of your company that is next time you raise." That's not maintaining percentage stake; it's actually agreeing to the possibility of a fairly small stake.
It's a non-trivial amount though -- it's probably the case that the next funding round for most YC companies gets low millions. So that's a nice chunk of the round.
Getting it upfront is unique though, and really quite valuable at this early stage. Still curious if YC will allow opting out -- I don't think I would have -- but still curious.