Likewise they may have certain legal rights if an employee violates their computer-usage policy -- but that shouldn't give them the right to violate the CFAA.
Checking whether an employee "completing their professional tasks" and reading their private correspondence are very different things. How is this any different (aside from prefixing the word computer) then if you had personal letters on your desk and a manager confiscated them from you (and read them) - in order to "confirm" that you were using "completing their professional tasks".