I have to admit that I still disagree with you.
I have about nine engineers working for me. I appreciate the work they do, and -- as someone who's a vastly less qualified engineer than they are -- deeply respect and admire their skills.
At my company, my job as a manager is defined to be all about attracting and retaining great engineers, and giving them context (and then they figure out what they're going to do with that context). So retaining them is, quite simply, my job.
That said, these engineers don't _belong_ to me or my company. They're human beings, and if I want them to work for me I should be willing and able to compete for them every. single. day. And that means that I don't win by making it harder for them to know what's out there in the job market that's better than the job they've got here -- I win by making this job the bast damned job they could want.
Trying to keep recruiters away from my engineers as a way to have a lock on them feels oddly similar to Apple suing Samsung to not have their competing product on the market.
I think we are closer than our comments appear. To me, it's not that you are keeping recruiters away from them; it's simply that if someone is hired to fill a vacant slot for you their loyalty should be to you and not use their position to your detriment. I would expect a good recruiter to learn your staffing needs, what current skills you have on staff, etc. to find the best fit candidate. For that recruiter then to go after you staff seems to me to be a conflict of interest and it would be unethical for them to tell someone else in their firm to go after them as well. however, I would think it perfectly OK for a recruiter at the same firm to independently reach out to your talent and try to poach them.
It's the same as what I do in consulting. A client has every right to expect I won't use what I learn about their company to help a fellow consultant at one of their competitors and it would be unethical for me to offer a competitor information on them I have learned while working for the client. To me, it's a matter of putting the client's interests first.
As a manager myself I fully agree with your comments concerning a manager's role and would not want to be a roadblock to their understanding the job market or having other recruiters contact them.