I've known runners who have taken part in net-downhill races because they are popular, not because they happen to be elevation-aided (the Boston Marathon is one great example). And in New England, the Grand Prix 5K for the year was on a downhill course, so in some sense a lot of good runners had no choice but to show up and run BS times.
Then there are runners who might seek out a downhill course as a one-off or two-off, either for fun or to qualify for a race they might otherwise be unable to get into.
The runners in these categories are typically very up front, even jocular about their times on such courses and don't count them as personal bests, are at least qualify them as aided.
In an altogether different category are those who not only consistently run on downhill courses, but count the times there as personal bests, AND do their best not to even let on that these times are elevation-aided.
In looking at the few runners I have known who fit into this odd category, they have a few things in common. One is being far better than average but nowhere close to "good" (say low 16 mins for a male on a "real" course). Another is being deluded easily and falling for scams like the worst of naturopathic or homeopathic medicine. Still another is acting like a high functioning autistic person. Yeah, I know, most runners are "a little off" but I'm talking waves of weird coming off these folks.