True, but psychological domination can be pretty bad too.
I enjoy games but I can't imagine how bad it would be to be forced to play a game (a game I don't even like) for an inordinate amount of consecutive hours! Even then it wouldn't be "playing" it would be slavery.
On the other hand, these are criminals serving their sentences in a prison, so it could be part of the punishment. As long as their human rights are not violated I don't see a problem with tasking a convict, however when it comes at the financial benefit of the warden/guards questions will be raised about the ethical validity of what their doing.