jaer said:True, but once charmed, someone is easy to shackle. Once imprisioned, someone is easy to break. (snip)
There are more ways to dominate someone than the spells with that name.
Well, that description and powerset alone does indeed give worldbuilders the choice to cast Mind Flayer thralls like that. They could assume, like you did, that Mind Flayers' slaves aren't actually fully mind-wiped, but just enslaved by terror and the superior power of Mind Flayers. That's actually rather cool. The "problem" for those of us who like monster stats to represent what monsters are capable of is when the choice is made explicit that Mind Flayer thralls are actually Dominated in some way - perhaps they have no minds left at all anymore and fall dead when their master is slain - but yet no information is provided on how this status comes about.
A little phrasing explaining that, for instance, Mind Flayers create their Dominated thralls by sucking out their target's brains and replacing them with magical goo, or through a five-minute version of their Charming process that can only be performed on a helpless foe, or whatever, goes a long way. It's good to let DMs make up how and whether monsters fit into their setting. It's bad to force DMs to make up stuff in order to explain how a monster makes sense or does its presented schtick at all.