Oh, I thought you were the original poster I was replying to. Sorry about that. Regardless, it's not actually all that complicated at all. Catching all fraud is almost impossible. Catching 90-99% is dead simple. If you don't like my quick and dirty breakdown, make another - the problem's size needs to be 4 orders of magnitude larger before the cost implications become a concern.
That said, there's a very consistent pattern at play here: Organizations and groups that refuse to take action on the basis that they can't catch everything are consistently those that intentionally look the other way in respect of fraud, and digital advertising has a bad rep in this area for a reason: most networks ignore as much fraud as they can get away with.
In any event, locking out other networks doesn't change the fact that the excuse provided for exclusivity is about how many containers are around, not how many networks can bid on that inventory. The rationale for the illegal activity doesn't address the problem at all.