It seems that some less than scrupulous persons have taken an interest in the fourth Humble Indie Bundle.
Prospective purchasers will now note the addition of a footnote stating that “only purchases above $1 are eligible to be redeemed on Steam to prevent abuse.” For the reasoning behind this move you have to consult the Humble Mumble blog which details how Valve’s current foray into gamification, The Great Gift Pile, has resulted in a spat of $0.01 purchases by those seeking to “legitimize” multiple accounts on the cheap in order to game the giveaway system.
Obviously this kind of abuse doesn’t do any favours to legitimate entrants or the participants in the Humble Bundle so the decision has been taken to not include Steam keys in any purchases under the dollar mark—but support staff can still supply them on request to legitimate buyers on an individual basis by request.