Hopefully I can provide some clarification on this issue for some of you. Etsy's policies state that each listing must represent a tangible item for sale and should not require the purchase of another item. This affects listings for things like deposits, shipping upgrades, add ons, etc.
Sherrytruitt is correct; part of the reason for this is that it could artificially inflate sales, feedback, etc, and could also become messy if a dispute were to arise.
I do want to stress that we understand that this is a concern for our sellers and, as Camilla mentioned, we hope to be able to provide a solution to this down the road.