Yeah, this is ok, but how does this iPhone behave differently than the usual here? Do the other phones (when acting as hotspot) keep broadcasting the WiFi signal when receiving data over the cellular network?
Of course, it's worth noting that tethering isn't a "service" that carriers have to build or support. It's an inherent capability of a sufficiently advanced computing device that has access to two networks simultaneously. But in a textbook case of non-neutrality, AT&T (and others) prevent you from using that capability unless you pay them to remove the entirely artificial restriction.