However, as with all methods of doping, you can't just turn up in China or North Korea [1] and demand to have their athletes tested. They'll just 'lose' the WADA officials passports at the airport until the athletes can finish their training camps in the region.
And it doesn't even matter if you test clean at the competition then, if you're able to dope completely unbothered during 9/10ths of the year.
[1] By no means do I claim those are the only 'dirty' nations
Of course the athletes trom those nations will go to the olympics, just look at how they're going to competitions, just to provide a urine sample, not even participating. Of course 'clean', to current testing standards at that point in time. It's also extremely useful to have your national anti-doping agency just help you improve your detection-evasion game, determining the limits of how much, what and when to consume the drugs.
They don't get tested when it matters, just at times they can basically choose, it's not much more than a intelligence test for their team doctors at that point.
And it doesn't even matter if you test clean at the competition then, if you're able to dope completely unbothered during 9/10ths of the year.
[1] By no means do I claim those are the only 'dirty' nations