It makes sense for them to charge for this service because checks cashed by non-customers have an unfortunate rate of fraud. If you're a customer, and the check turns out to be stolen/forged/lost/written to someone else, then the bank just reverses the charge. If you're a random guy who walked in off the street and walked away with a pile of perfectly good cash, then they've lost not just the time in providing the service, but also the cash.

There are other places that provide check-cashing services. None of them are free.