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.
Valve fucked up but the Verge still broke the social contract regardless of whether they're legally in the clear or not.
Doing something just because "it's legal" doesn't make it a moral justification. My wife and I have a joint bank account. It is legal for me to take money from it and gamble it all away, the gate is "open" but that doesn't make it morally justifiable.
Meh, I don't think there's anything morally wrong with what he did. What he did wasn't just legal, it's literally his job. The only issue is that Valve is now angry at him for their own failing.
To continue the same analogy, they didn't just leave the gate open, they literally invited a bunch of people and told them to invite other people. I'm not sure what they expected if not this exact situation.
Valve isn't really angry as far as I can tell, or have heard. They're about as angry as any other person which goes and posts this stuff online: revoking access. If Valve wanted to expand their testing userbase without people leaking it online, they would have sought NDAs and other legally-binding agreements with testers and - by extension - journalists who can test the game.