They do, it’s extra, and NOT unlimited. That’s where they get you, the same way they had limits on texts and pics back in the early 2000s. (AT&T knows they have to make their money somehow and right now it’s on data caps.)
However, older contracts with AT&T are grandfathered in to allow hotspots with their plans, even if they’re unlimited. The two catches are that they’re 1) obviously grandfather claused and therefor not available for new contracts, and 2) if you change your plan it’s expunged. Getting a new phone that requires a different plan can also void the clause.