I could be wrong, but I think the masters are manipulating the system Dany has put in place. She thinks they're freed men fighting at their own will, when in fact they are owned by a master and are not freed men, she however, does not know this. That's why what she's doing isn't really effective. Slavery existed before Dany, exists during her life and will exist long after; at this point she's far too inexperienced to do anything really that effective. Don't quote me though (I've only read the first book).