The FAQ merely states it. There is no reason that I could see. Could it be to let enough people see the question in the hope that someone would be willing to answer without the incentive of a bounty? Or is it some precaution to prevent the system being "gamed" in some way?
Assuming that someone can answer but is holding out for a bounty, shouldn't the asker be allowed to offer a bounty sooner, even immediately, if the asker wants a speedy (and satisfactory) answer?