I'm pretty sure I've seen defenseless receiver calls for hits like that because that can cause head and neck injuries when landing on the ground. But idk for sure if that's still a rule or whatever.
I believe it is a "strike zone" in the upper chest and head, and hitting the legs is legal. The intent of the rule is to eliminate forcible hits to the head and chest while the receiver is trying to catch a pass and not focused on an incoming defender.