There are things that are legal between parties of (presumed) equal footing, that aren't legal between employers and employees.
That's why you can pay $1 to buy a gadget made in some third world country, but you can't pay your employees less than say $8/hour due to minimum wage laws.
Being paid a whole lot of money to not talk about something isn't remotely similar to paying someone a few dollars an hour. It's not morally similar, it's not legally similar and it's not treated similarly by anyone who deals with these matters and has a clue what they are doing.
That's why you can pay $1 to buy a gadget made in some third world country, but you can't pay your employees less than say $8/hour due to minimum wage laws.