So, in the other-branded vacuum case, I can see a reason: buyer's regret.

Chances being that anything that irks you about your new vacuum are going to be most accutely felt in the initial period.

So reminding you heavily that there are other, greener-grass vacuums out there seems productive in inceptioning "I'll never buy X brand again!" resentment.

Yeah but you can turn that argument each way, it's just reasoning backwards. It amounts to just doing whatever with the tracking data, waiting for something positive to happen because of it, and then claiming it.

They can do that with random data and it'll work just as well. It's called divination and you don't need to track me for it.

Amazon doesn't care if your loyal to the vacuum brand. They just want the purchase to be through them. If you are unhappy with the purchase they want the rebound sale.