In Valve's FAQ about their free-to-play model, they specifically said they will monitor and quickly deal with people who try to cheat the loot system with free accounts.
Also, there are a few restrictions for free accounts in that they are unable to trade away stuff they get, they can't get cosmetic items, and they are limited to having a lot less slots to store items.
One purchase of anything from the store makes the account fully functional, so if someone wanted to have 20 idle accounts getting free items, they'd have to pay Valve something on the store to activate premium status on each one.