No.
He/she is asserting that we do not yet KNOW what they are.
And there IS a legal definition of polygamist. If they do NOT fit that defintition then they are NOT considered polygamists under the law. And the LAW is all that should matter to the State. Unfortunately an alarming number people, like you, get LAW and MORALITY confused.
There is not (and God help us I hope there never will be) a LAW against everything YOU consider immoral.
As somebody else explained, I guess there is no way to charge them if they don’t actually have real marriage licenses. Calling themselves polygamists is not enough to charge them even. They have to have the the real licenses.
In which case, there really is nothing that can be done unfortunately unless it is proven they abused the children.