If it's anything like e.g. financial planners (and I'm not sure that it is, but I have some familiarity with similar situations in that field), when he moves to a new firm he isn't allowed to inform any of his former clients, and the old firm won't tell them where he went, but any clients who figure it out and can procure his new contact info are welcome to transfer their custom to the new firm.
If you have clients that are willing to switch firms to stay with a particular person, is that person not just as much part of the product as the financial instruments hes selling? If my doctor switches practices, but he knows my medical history, is the doctor not as much part of the service as the medicine itself? Would it be appropriate for a practice to keep that doctor from working as a doctor somewhere else because of competition?
You have to balance that with the fact that the doctor has the right to stay employed. If contents of the doctor's brain (including the medical history of his patients) are so critical that the practice would lose customers if the doctor left, then instead of pursuing legal action against the doctor if he leaves, the practice should ensure he is compensated enough that he has little incentive to leave.
Well in fact financial planning is in large part a snow-job. (The parts that aren't "invest in index funds", that is.) So the greatest actual skill is customer relations, which is an inherently personal thing. It's not clear that IaaS is equivalent, but maybe that's what Amazon are implying with this lawsuit.
