They ARE justifying it. They said that polygamy is pretty common and legal in that continent/World like if it was general knowledge so there wasn't really a need to make it look like is a bad thing or even mention it, yet they felt the need to explain that is OK.... that literally is the meaning of justify... they are justifying it to the reader, and that why I said that is unnecessary specially from a setting not tied to real world constraints....
Okay, better to say they aren't "over justifying".
Like, think about it... if you don't say that, then how will readers know harems are allowed???
If anything, it's necessary to let readers know the option is available and that the "marry everyone" route exists.
Like, think about it. Plenty of fantasy stories don't have harems and not every MC can marry everyone.
A single line clears up any problem that could exist.
Like, why does it being a fantasy mean that they don't need to state it??? You can have a fantasy with a legal harem or not...
Giving the line clears up and lets us have the fantasy that the harem is allowed...