In BTC & ETH, the fee disbursement is part of the block the validators are getting consensus on. In the PPJ (Promote, Propose, Join) scheme, the fees depend on the consensus process itself. If we push the fee distribution transactions to another block, what happens when we rotate validators? We have interested parties leaving---potentially all of them!---and uninterested ones joining. What enforces the correct distribution of fees?
Vlad suggested that we could write a contract that would look at the blockdag and disburse the fees, but that would require rev itself to run.