Just trying to learn, here: what happens if most of a given niche is involved in buying/trading links? Wouldn't the norm for your niches then NOT represent natural link building, so a natural link builder might get mistaken for a spammer? Or do you think Google has ways of filtering for all that?
1.) When dealing with the numbers they are, almost anything is possible in some cases. This is one of the reason the answers they give really have to be so "noncommittal". I would guess even if you pointed out specific paid links on a random page that were obviously paid to a Google Engineer who works in that area and asked if those links count and you got a "definite response" it would be something along the lines of "they shouldn't" and my guess is if they expanded on "they shouldn't" it would likely be "they shouldn't, but I'll have to check to see for sure..."
2.) I think they try to safeguard against that (have a way of filtering) especially over time
and will likely get better at it in the future especially as more time passes
Penguin was supposed to be a "playing field leveler" of sorts and even though people may think they have "natural looking paid linking" (or even link building in general) figured out there are probably a number of factors that over time will show built links are not actually natural, and "sheer numbers" may actually indicate "more unnatural" even if it's for most sites in a niche than the limited number of sites in a niche going "completely natural" and allowing links to happen without building any. (Feigning natural/random is much more difficult than I would guess many think, and over time
my guess is a non-random (not natural) building "scheme" is likely algorithmically detectable.)
I don't think they get it right with the safeguards (filters) I would guess they have in every case and obviously they don't hit every site that's building links (paid or otherwise) right now today either, but I would think part of that is because it may take quite a bit of time for a pattern of unnatural to emerge and be apparent enough to be algorithmically actionable, but one day, eventually, when the pattern of "unnatural linking" shows up we have a webmaster who joins WebmasterWorld because they don't rank any more and Google's broken because they have the most links and all the right anchor text in all the right percentages and they have obviously out SEOed everyone else in the niche and the other site(s) that are outranking them don't have anywhere near as many links and some haven't even built any, so there's no way they should be number one, except that Google's broken.
(Does the preceding sound increasingly familiar from recent posts? I think one thing to remember is Google generally has a much "longer-term" goal than most webmasters seem to and I'm also fairly certain I remember reading more and more of those type posts recently. I would guess we're seeing the results of the "filtering" you're asking about more and more frequently, but that's just a guess/opinion, so I could be completely wrong.)