The FDA is concerned about coercion. We find this all the time -- someone wants to do something voluntarily. Someone else doesn't, but gets coerced into it through some sort of blackmail and forced to claim to be a volunteer. The only way anyone has found to block the coercion is to block the volunteers.
You see this all the time in work contracts for unions -- the union wants to stop business from demanding ridiculous overtime, so they negotiate limit X. They then include a clause that bans anyone from volunteering to do more than X because if anyone can volunteer to do it, then you can be coerced into doing it through various economic pressures -- as a manager, you'll promote/retain the one who volunteers more of his time, thus making it effectively a requirement of the job to volunteer in order to stay competitive, so you're right back where you started. The only solution is to ban volunteers.
Same with FDA.