Even when charmed, heroes have fatigue.
On what are you basing that conclusion?
If I had to guess it's the same thing I'm basing it on, which is the fact that nowhere in the rules are monsters "required" to have no stamina value. It just so happens that monsters are designed without stamina. The hero isn't, and nowhere does it state that his stamina disappears when he becomes treated as a monster (or that any of his other attributes change.)
It's very similar to how monsters (in general) auto-fail attribute tests because they don't have attributes. However, LT's do have attributes so they don't auto-fail, yet they're still monsters- they're just a special type of monster.
Edited by Zaltyre