Aren't they already illegal? (ref to the infamous Jobs/Google email chain).
A non-solicitation agreement is not strictly the same as collusion between companies to agree to not hire away each other’s employees.
If I sign a non-solicitation agreement, I as an employee cannot move anywhere else and try to hire former coworkers for some period of time.
The scandal that happened among multiple tech firms (not just the ones you mention) was that several companies agreed at the top level not to recruit and/or hire one another’s employees.