> Amazon (or whoever) showing you the same stuff you just bought is kind of a separate issue, because it obviously isn't useful to anyone (including Amazon) and surely is so easy to avoid I find it bizarre that it still happens in this day and age.

I've asked people at Amazon on the recommendation algorithm side about this, since it comes up so often in discussions.

Their answer is that, surprisingly, it is a good heuristic. If you just bought something, you've shown intent, and there's a small probability that you're going to be unhappy with your initial purchase and back on the market soon, for whatever reason.

Is the posterior probability that you'll buy a mattress again shortly after buying one, a small chance? Yes, undoubtedly. But is it smaller than the prior probability of a random person who hasn't bought any mattress at all recently, suddenly being on the market for a mattress? Apparently not.

Weird things can happen with joint probabilities when both sides of the condition are very unlikely. Sort of like how your chance of having some rare cancer after testing positive on a ~99% accurate cancer screening test is still quite tiny.