I think its more to do with obfuscation than simple agent buying stuff. Perplexity disguises its agents as chrome browser, and that messes up things at Amazon's end. Not saying its good or bad, but think of ad relevance, then recommendations effectiveness, placements etc. because by default an agent is going to click more and more randomly than a user.

Then, when the agent buys stuff, my guess is that returns are a higher %age (because naturally agents have no preference model so end users might not like what they purchased eventually and returned). I don't know if enough people returned for it to matter, but as a user, if an agent is making a purchase and i know its super easy to return, i won't check everything agent did, just buy and then return if I dont like it.

I don't think it's always a price comparison or margin thing at this point. Given their margins and volume, the agent purchases woudl need to be an order of magnitude more to matter on that front.