If the password was voluntarily revealed because you want to be employed and are ready to provide it as a condition of employment, I don't see how you can call it "unauthorized access".
Volokh discusses something else - what if the user does not allow the access voluntarily but is forced by the court (which, unlike employer, is entitled to use force to compel people to do things) to reveal the password. Then it would be like breaking into a house on a search warrant or forcing you to open the safe (this was discussed some time ago here because of other court decision that said - in TLDR version - that 5th amendment protects passwords). But that's different situation.
Exactly. Imagine if you had a "Million dollars for the login credentials of FBI/CIA/DOE employees!" program. Just because the employees want the million dollars, does not mean the payer is authorised to access those systems.
OK, I guess you have a point, if the information there is not sole property of the password holder - especially as in the case of FBI, where FBI employee has access not to his collection of lolcats pics but to something more important. I guess one could argue on Facebook you can access other's information too, so it still applies.
Nobody has rights to hold particular job, it's an agreement between employer and employee. If the employer requires you to wear a suit as a condition to employment, you can choose either to comply or keep the jeans and tshirt and find another job, both decisions voluntary. Same with other requirements. Some of course are not legal - so, unless you work in very special industry, having sex with coworkers is not a legal condition - but I don't see Facebook mentioned anywhere there.
Volokh discusses something else - what if the user does not allow the access voluntarily but is forced by the court (which, unlike employer, is entitled to use force to compel people to do things) to reveal the password. Then it would be like breaking into a house on a search warrant or forcing you to open the safe (this was discussed some time ago here because of other court decision that said - in TLDR version - that 5th amendment protects passwords). But that's different situation.