I see nothing wrong with it, actually. People want — and have a perfect right — to know, who they are about to trust with powers over them and/or their businesses. And the higher the position, the greater the powers and, consequently, the greater the extent people might go in their investigations.
The "opposition research" is just another facet of this. If it is legitimate for all of us to study, how Donald Trump parted with his ex-wide 30 years ago before we hire him, it is certainly legitimate for a would-be employer to check criminal history of a candidate, or inquire, whether he has done something, which may betray certain things about his character or judgement. Did he torture animals? Is he prone to binge-drinking? Has he burned the national flag? Is he a racist, sexist, or communist?
So long as private employers' hiring decisions remain their own, they ought to remain free to base them on whatever considerations they please — with the specific (if regrettable) restrictions imposed by the law, of course.
Well clearly I'm not going to have such studies to hand, not sure how you would study such a thing
Well, you made a wide-reaching statement about a certain fact. If you can not cite anything to confirm the fact, your statement remains unsubstantiated and the "fact" — highly suspect.
there is inbuilt racism / nationalism in CV selection
I can believe that — and in my not-so-humble opinion, those concerns ought to remain up to the employer as well. Both from the principled standpoint — being free must imply freedom to be wrong, as well as practical — the war on thought-crimes, waged in this country since the 1960-ies, is even less winnable than the coterminous war on drugs.