The failure mode for this theory, of course, is if one of the companies -- especially a service like Squarespace -- either shifts their business model to something slimy or is revealed to have been doing something slimy all along. Personally, I don't think Gruber would hesitate to both report on that and stop taking their ads -- doing so would have little to no effect on his bottom line.
(The observation made by another commenter about how many bloggers and, of course, podcasters pump Squarespace without actually using Squarespace is wryly amusing, though.)
I'm pretty sure that if they're in anyone else's good graces, it's because they provide a great product with solid service at a decent price. It's absolutely not because they had to bribe people to overlook the fact that they weren't really any good.
In essence, DF is an endorsement-based model. This is one where a justifiably trusted source uses their hard-won credibility to promote products that reinforce their credibility. It's pretty much the opposite of a model that allows people to "buy their way in". Indeed, that's the whole point. You can't buy your way in. If you don't have a decent product, you're not getting listed.
I don't think there is anything extremely unscrupulous going on, just that there isn't much of a wall between editorial and advertisement with Gruber's model as some people claim.
[1] http://daringfireball.net/feeds/sponsors/2014/01/cleanmymac_...
Just wanted to second your comment and add that Everpix too was also a really good product before they closed down (I still haven't found an alternative that works for me).
1: http://daringfireball.net/linked/2014/08/10/ios-at-google
2: http://daringfireball.net/linked/2013/04/20/windows-azure-mo...