Another difference is that steriods at the level taken by athletes use is demonstrably harmfull for their health, this is not necessarily true for mental boosters.
Uhhh, professional sport isn't 'real' work? I would would say without a doubt every single processional athlete probably works much harder then your typical white collar worker (due to the fact that they are not typical athletes) As far as "productive" goes, each pro organization is staffed with hundreds of people all doing 'productive' work which is possible because of the athletes that work for the corporation.
Now contrast this with mental abilities. If people can solve medical problems then they can prevent deaths and enhance health. If they can solve technical problems they can make peoples lives easier and allow them to achieve more. In general mental skills can make the world a better place. Not a zero sum game.
In contrast, regular "productive" work is not trying to be entertaining, it has other goals (e.g. moving packages from one place to another). If I ride my bicycle while delivering packages, more packages get delivered.