While they did manage to find something to check off the right forms as "is criminal", those were years ago, and had no reason to do anything about it now, and no reason to suspect he would reoffend, given the nature of the crimes.
So no, they're in the wrong.
Seems a little threadbare as a theory.
People have been adding other people to royal families for the entirety of recorded history.
Sometimes its through marriage, but sometimes its adoption, sometimes they just make up a lineage.
Now, theres arguments against royalty, for sure, but if the royal family wasn't allowed to prune itself, find the best people and merge them into the royal family, etc, there never would've been royals in the first place. Royal families begin with individuals but they remain by caring about "good breeding" (and other ways of consolidating power).
Consolidating is the real purpose. It can be obscured with religious lines of divinity, or what have you, but royal families are always shopping for people to incorporate.