If I recall correctly, it's a holdover from the video game bundles they offer. Initially you could just plug in $0.01 and get their bundles for a penny each. They naturally would have trouble because of card processing fees, but they allowed it until shortly after they started giving out steam keys for games. A "security" mechanism that steam has in place is they don't allow users who haven't purchased any games to participate in trading items/playing the steam marketplace. Scammers were using humble bundles as a super cheap way of getting tons of keys they could use on fresh accounts so they could be used to scam items from legitimate steam users.