It's good to question things like this. What is the hidden cost of transferring Hive Engine tokens? Hive Engine Tokens transactions are implemented via Hive transactions. Doing a transaction takes up resource credits (RC) of the user who issues them. You can run out of RC, but RC recharges with the amount of Hive Power you have.
In Hive, a transaction includes more than sending tokens, it also includes any way you write to the blockchain: including posting and voting.