From my understanding users of the beta can then invite others to join as well, Valve isn't necessarily directly choosing who has access. So if Valve didn't send the invite themselves they wouldn't know to specifically put someone under a more strict NDA or whatnot because they're a journalist. Could they have done more to restrict all users from sharing information? Yes, since apparently you just have to hit escape to bypass the agreement pop up, and there's no other sort of NDA or contract or w/e in place upon joining.

I'm just speculating, but I think they chose not to do that so people could openly get their friends playing with them instead of going through waves of sign ups and hoping to get in together, or otherwise risk people losing interest when they can only play with randos. I could also see a line of thinking where you assume people want to talk about the game, so let them bring others in to play with them and that gives them someone to talk to about it too instead of just spilling the beans for randos on the internet.