The employer might state that "it's our mutual understanding" about anything in job offer, but it's not a legally binding statement. Anti-competitive statements can be binding, though, so it might be illegal for them to work at FB and Google, as their business might intersect. But it's a different story than just right to hold two "full-time" jobs.
One possibility I might think of -- if an employee is really a high performer, but couldn't negotiate compensation above "market price" for their level.