Ebbie, I suspect it is to preserve the genetic integrity of the breeding line as in if a cat has only had certified pregnancys from provable sires than the parentage of the kittens in all cases is verifiable, this of course is probably difficult if your mog has any degree of freedom of action whether Thoroughbred or plain moggie as they tend to spread it around a bit given half a chance.
On the scientific side altho' genetic material does not enter the mother cat from the kitten there are things in the shared bloodstream that are "trans-placental" such as antibodies that in humans are connected with blood grouping issues for example blue baby syndrome that is triggered by the antibodies generated during the first pregnancy (if the parents have certain differing blood groups) that attack the fetus during the second pregnancy that could be seen as contamination of the "pure blooded" breed of cat.