I think at a certain point the league became so big and access became so important for these guys and their publications that reporters naturally started becoming more and more careful.
Look at it this way - you’re a guest coming into someone’s house; teams have no legal obligation to allow you into their home and don’t have to take your crap if they don’t want to. Want to be the tough guy pain in the butt reporter every week? Ok great, but guess what, you won’t be getting a credential next time.