they just pay the fees themselves?
Yes. It's a marketing thing.
If you pay the user transaction fee, he will start using your service and recommending it for his friends. And having costumers is something more profitable in the long term than saving a few bucks in fees in the short term.
Seems odd if they do! Is there anywhere else that does that?
I have seen people saying that Gemini does the same. But I can't confirm.
Other than that, it's not profitable for other services to do the same and pay the fee for the users.
