Well, first off, “free” is not a requirement for a 3rd place, but even if we accept that the best third places are free, that doesn’t make ones where money is involved bad by default.

The difference is between paying admission or membership dues like gyms usually require vs a public space where there’s an expectation you will buy something but it’s not a requirement for entry, and it’s not the only thing to do there.

Stores don’t work because the purpose of being there is to buy things, so there’s nothing else to do, and no other acceptable behavior beyond maybe some chit chat.

But at a cafe, the seating area is designed for you to just chill and do other things besides ponder what you’re going to buy next. The seating areas are open and there’s an expectation a purchase is made, but you can order nothing while a person with you orders something, or you can order something small and cheap, and get the same level of access as anyone. That’s a very low barrier of entry for a place that is purpose made for social activity.

Same is true of bars. You need to buy something, sure, but the place is designed for social activity, not just reading the menu.