Iota doesn't have free transactions. The payer has to include proof-of-work. Proof-of-work costs some electricity and/or CPU resources. Also the centralized servers oftenmay need to resubmit the payer's transaction with new PoW, as admitted by Come-from-Beyond in our discussions in the Decentralization thread. They will not do this PoW for free forever. Iota can I guess afford to subsidize these servers for the meantime to give the illusion you desire.

But sorry the correct answer is there will be never be transactions without a fee. The fee will always be charged indirectly if not directly, e.g. Facebook is free to use, but you get bombarded by ads to pay for it.