ELI5: Rootstock fees to miners origin

Rootstock WP mentions that due to merge mining, it incentives miners to mine it to get extra revenues in forms of fees. Page 5 of the WP says:RTC is the base currency used on the RSK sidechain to pay miners for transaction and contract processing. There is no currency issuance: all RTC are created from Bitcoins coming from the Bitcoin blockchain.and then in Sidechains post it is mentioned that:The “transfer” is in fact an illusion: bitcoins are not transferred, but temporarily locked on the Bitcoin blockchain while the same amount of equivalent tokens are unlocked in a secondary blockchain. The original bitcoins can be unlocked when the equivalent amount of tokens on the second blockchain are locked again (in the secondary blockchain).When no currency is issued and all RTC comes from locked bitcoins, where do the fees for RTC miners come from? Because if the release of locked bitcoins is waiting for the same amount to be unlocked again, how could you use part of that amount to pay as mining fees? via /r/Bitcoin http://bit.ly/1TOCNMF