My view on this is like crime but with a slight difference.
"If you have done the time" then you are entitled to a fresh chance to prove yourself in society.So yes, they should. My rider is a medical one though - if there is any evidence that their body and mind have benefited from taking such steroids or whatever and their current performance could be enhanced, then no - permission to re-join should not be allowed. I suspect that this might just correlate to the "severity" of the drug taken as well as to its impact on the body.