From Amazon's perspective, someone who just bought a toilet seat is much more likely than the average person to buy another one. Maybe they're doing interior furnishing work and put loads of seats on toilets. Maybe they're replacing a broken crappy plastic seat on one of their toilets with a nicer one -- might they perhaps decide to upgrade their other toilets as well?

And sure, I know that these are all pretty weak signals and that the toilet seat recommendations will probably go un-clicked. But Amazon has to do the best it can with not very much information, and if they have to hawk toilet seats to a hundred people who've already bought them to find the one guy who's remodeling several bathrooms, that's probably a really good gamble for them to take.