I am not in this field, but I guess that as an employee at the FDA you approve drugs for all the big pharma companies, which aside from the FDA are the major employers that value their skillset. It may be that if you prohibit FDA employees to transition to one of the big pharma companies, you're telling them that they can either work at the FDA forever or have to leave their field
"Take a job with the Fed, sign up for a lifetime of lower income so that HN can nod sagely about avoiding the appearance of impropriety" is a recipe to get no one to ever work for the government.
