If an individual had somehow done this, I expect that the Computer Fraud and Abuse Act would be used against them. With Meta, we'll see.
I heard about this a few years ago. The trial participants were informed, consented, and paid. If you consent to a root cert being installed and analytics being proxied, well, that's that.
Saying we’re going to do “ traffic monitoring” doesn’t carry the weight of “we are going to listen to your private conversations”.