Why aren't the private transactions free aswell?
A very interesting question - thank you for bringing it to our attention.
Firstly, to understand it better we must delve into all that is represented by the Delegate Proof of Stake (DPOS). Simply put it is a protocol where all the users (owning coin) delegate their vote to a representative. The vote is then weighted by the balance of the wallet. In the end the representatives, which act like the minors to maintain the network, will validate the transaction whereas at least 51% of the network must validate a transaction for it to proceed. The 51% is determined using the entire portfolio of wallet balances.
Hence having a private wallet means that the balance will not be displayed. The DPOS will therefore not function. To bypass this issue we have develloped a public wallet and an alias private wallet. The public wallet delegates his voting rights to a representative as the balance is public. Take into consideration we now have a public and a private wallet.
Lets say there are no fees. Its clear all users would prefer to enjoy the privacy feature and in turn keep their coin shares in the private wallet- yet this would lead to the issue of running the DPOS off track as the representative would no longer be legitimate as they would represent a very small part of the network. Therefore the fees in place are an incentive to make people keep their coin in their public wallet and run node.
Furthermore, 1% of the fees will be used to remunerate the nodes, and 99% of the fees will be paid back to the network depending of their number of shares in coin. Hence, in average, if the number of transactions are higher than the number of user, each user will get back 99% of the fees paid. The 1% remaining will allow a more decentralized network for both public and private transactions. On top of that, public transactions remain free of charge, and the fee for private transactions will be very very low.