Again, you see these people as "the places they work for".
He wasn't "handed a position".
He applied for a job, got interviewed, and took it. What favor do you think happened here, or anywhere? What evidence do you have of this?
No, i can't see a general problem with antitrust lawyers, or really, any lawyer deciding where to work.
Private law firms hire people with government experience precisely because they know how it works. Not because they want favors, but because they have experience. The same is true of Google, Microsoft, Facebook, whoever. This is no different than hiring from "industry". The industry of antitrust lawyers includes government lawyers. You try to hire the best, regardless of who they work for.
If your goal is to prevent favors from being done, prevent favors from being done[1]. I don't see why that has any general relation to job mobility.
People either have integrity or not. Either they make decisions and voice opinions without regard to future career mobility or not.
If they generally don't, you are pretty much screwed no matter what you do here - you need to fix people, you aren't really going to fix the process[1].
If they generally do, than you should just worry about the outliers..
[1] Imagine for a second, you go so far as to have a neutral committee review every person who changes jobs, including all correspondence, video tapes of interviews, whatever.
Do you believe this would fix it?
No, of course not, it'll just drive any bad behavior further into the dark. It would literally not stop any problem.
If you ban job mobility, would that stop any problems?
Given the number of senators/congressman/etc indicted for having their houses renovated for free, etc, i'm going to go with "no".
If your problem is integrity, people without integrity are going to be much more creative about the favors they get, how they are done, etc.
Changing job mobility will not change this.
As evidence: It literally hasn't, in the past 200 years.
The only thing it does is hurt the honest people.