Affiliate revenue. The same reason they recommend all the other products on their site. It's advertising disguised as product reviews. You are a sucker if you believe anything an Amazon affiliate is telling you.
I have found their reviews very extensive, more than any other free alternative that covers so many product categories, and typically they match what you get if you purchase one of the products. You'd think having NYT behind them would lend some legitimacy, at least as far as it concerns outright fabrications. Typically I don't mind if their recommendations are sourced from just a handful of retailers in that case.
There are a dozen other humidifiers they can earn affiliate money through.
I have a low-end "Victsing" model and it works just fine (as evidenced by the hygrometer next to it), and isn't hard to clean. It isn't a complicated device to make.
All that is to say, that's quite alright with me.