> [the aliens] wished to prevent mass hysteria
I don't think any government agency has a right to hide the existence of aliens from the public for the public's good, whether the aliens consent or not.
Proof of the existence of aliens would impact our understanding of the universe and ourselves so fundamentally that hiding their existence is cruel.
It would be like teaching your kids not to kiss or they'll get pregnant. It's a misguided attempt at protecting people in ways that prevents them from dealing with real life.