@andrew: If you verified that the sponsorships were up to date than that sounds adequate. I guess I missed that in your article, but I was left with the impression that you just checked their pages, making the assumption that not being listed means that they don’t contribute.
Do any of the trail orgs you looked at list individual donors and their occupation? Dentists may be making personal donations as well and you’d have no idea unless their info is listed.
Although your last line makes it a bit more clear what the thrust of this article is: they’re missing a marketing opportunity. Up to them if that’s valuable or not really, but somehow I doubt it.