I don't know if there are federal rules here. I think this is a state thing.

Some (many? all?) states have agency laws. I don't know how it works, but I vaguely recall that, for example, in Texas an agent is required to present every bona fide offer to the party being represented. I'm pretty sure California caps the length of a agency and/or management contract.

The concept of a “manager” is different with different rules.

Even if the agents don't have to be licensed there are rules they are supposed to follow in theory. The laws are likely to be pretty well developed in California due to abuses in the studio system in the old days.