Do they check every machine? From TFA:

  > A technology expert at the ­Washington-based Brookings ­In­stitution, Professor John Villasenor, said the globalisation of the semi-conductor market has “made it not only possible but inevitable that chips that have been intentionally and maliciously altered to contain hidden ‘Trojan’ circuitry will be inserted into the supply chain.

So we aren't necessarily talking about every single machine being back-doored. My point is that Dell and HP aren't necessarily any more secure just because they're American companies. The home-country of the company whose badge is on the machine is pretty much meaningless in a globalized world.

The article also contains some suggestion that this was motivated by rivalry in addition to security, so there's that as well.

I see such back doors as less of a problem for the general public than it is for intelligence agencies because because even a single malicious device could result in a major security breach and so randomized testing mightn't be as effective. If trying to snoop on the general public on an ongoing basis though then it would be hard to avoid randomized testing (presuming someone is actually doing such testing).