If it wanted to do so, Google could treat links from sites is deemed to contribute to spam in any way it wished. It could easily create a list of the websites most used for such purposes and then apply specific rules to those sites.
It might be doing this already.
For example, knowing that you can buy posts on a specific contributor section of a high DA website it might ignore completely all links from that section.
More likely, it might treat them as no follow, regardless of the HTML on that site - with the effect that the anchor text is still taken account of, but no link juice passes.
Subdomains are, of course, treated as new sites, so web 2.0s that allow you to create a site on a subdomain don't benefit from the links to the root domain.
It is also easy to identify profiles and discount those if it wanted to do so.
Comments from Google's employees and representatives should always be taken with caution. Of course, Google doesn't want anyone creating content for the purpose of ranking. So it will say that such links don't work. But there are different levels of not working. Google may throttle the link juice passed through those links. It may stop all link juice passing. But it may still use the anchor text in its analysis of link spam. Or it might ignore the link altogether. That's why you can't discount the link type entirely - it may have some SEO benefit even if it doesn't pass link juice as effectively as other types of links.
Additionally, Google might be right about these types of links not being particularly effective, not because Google treats them differently, but because Google treats them the same as others.
The amount of link juice that passes seems to depend on two factors - how much there is on the linking page, and how relevant the linking page and the target page are.
Not many people link to profile pages at all. So there aren't going to have much link juice passed to them anyway (even from the parent domain). When people do link to them, they tend to spam them with large numbers of topically unrelated, low power backlinks. If Google doesn't get alerted by the link velocity to the spamming, then its still unlikely to pass as great a percentage of the link juice that it could pass on account of the lack of topical relevancy.