They're relying on the general public remaining ignorant of WD40's failings - assuming they're even aware of the failings themselves, which they might not be.
If things turn sour they'll silently drop the recommendation, I'm sure. Of the locks that fail, I'm sure they can claim all sorts of natural causes for the vast majority - and silence the minority quite easily.
It'd be interesting to ask them what sort of longevity testing - if any - they did on locks treated with WD40, but I can't be bothered - I've not seen any Yale products this side of the Pond, so it's just not an issue for me.
cheers
Jules