If the qualification of "random" in your post is the essence of the difference, then you're missing the point.
Giveaways/recruitments are open to whoever is willing to take a couple of screenshots and upload them to an imagehost. Run a speedtest. Answer a few questions maybe.
The difference in who is giving is not important except in the case where a site is actively seeking to stop public giveaways. So it only matters when someone like the OP is out there hunting and reporting the giveaway. It doesn't matter generally who gives the invite because they are already a member. Sure, an authorized giveaway has the express confidence of the staff, but it's who is receiving the invite that matters. It's the random user who receives the invite that is the problem.
I've signed up at so many sites through giveaways, recruitments, irc interviews, http application forms, even trades and friendly trades. I know from experience that those recruitment efforts are hardly more secure than the random user giving out invites because I've never been denied...