"small allowances" You mean tolerances? That has been discussed in detail here, the tolerances are likely no different than a civic from 10 years ago that was specced to run 10w-30. I'd also venture to guess that in other parts of the world, other grades are specified to be used in that engine at various temperature ranges.
It is because we have designed and enabled such good 20wt oils, that we can use them with confidence.
The cooling aspect may have some truth to it, but is for the most part irrelevant to why they use this oil.
The real reason is because people have been tricked into thinking that they NEED 200 hp. As a result, they have vehicles that are thirstier - direct contrast to CAFE and being "green", mandating higher fuel economy, etc. One way to benefit fuel economy and eek out each last bit possible is to run a lighter oil, which can yield a few percentage points of economy.
SInce as mentioned before, we have designed and specced 20wt oils to be so good, they typically can take the application without issue.