It's not impossible for that to be a clever decision by Amazon (although I'm not saying it's likely, I have no idea about the numbers).

The ultimate goal of the advertising is return on investment, not making you feel interested in the adverts. If, to exaggerate the possibility, 100% of "people who look at tech" are 0% influenced by adverts, but 10% of "people who bought a unicorn thing" will go on to buy another if they're constantly reminded that whoever they bought it for likes unicorns, all of a sudden it would make sense despite being counterintuitive to viewers.

A more commonly discussed example of a similar thing is that it's easy to think "I just bought a (dishwasher, keyboard, etc), I obviously already have one so why am I seeing adverts for them?" Sure, it might be that the company responsible has an incomplete profile and doesn't know you bought one already. But it's also possible that the % of people who just bought the item and then decide they don't like it, return it and buy a different type is high enough to be worth advertising to them.