> are literally funding themselves out of deposits
No, they probably are not. It’s illegal and money transmitters are regulated.
What they are doing is collecting interest. One tends to see less scrupulous transmitters hold funds longer when rates rise. I didn’t think Stripe fell in this category, but it’s becoming difficult to say.
> good one is business selling gift cards
You’re right. (Also customer deposits.) This doesn’t apply to money transmitters, however.
They are literally funding themselves out of deposits, in a small way if they keep any persons money for any period of time. If they start doing it to lots of people for months at a time then they are at least partially funding themselves from their customers.
Lots of businesses do this incidentally. A good one is businesses selling gift cards. It's not the same; but this set of conditions is not super uncommon.