A sidechain.borrows all coins from its host chain. So while it can not have its own block *subsidy*, it still has miners earning block rewards composed entirely of transaction fees.
It doesn't "borrow" coins, it pegs them to the host chain so it can have flexibility. It also has an entirely separate chain that can an entirely different consensus rules which means it could technically have block subsidy too although in a pegged coin it doesn't make much sense.