The way this works is, the content provider (actually, someone working on the content provider's behalf) does an independent check every so often.

Then they (the content lawyers) come back to Netflix and say "hey, 10% of these attempts got through, you are breaking our deal.

Obviously, some of these of these service providers are not the brightest bulbs on the internet, but try telling that to the legal department of a media conglomerate.

So, the technical team that handles this at Netflix has to shoot for the worst case, instead of the average case, even if they are already being effective.