I think the problem with this (at least on ebay, where it's also present) is that they rely on the seller to set the metadata, and the seller is generally more

I think the problem with this (at least on ebay, where it's also present) is that they rely on the seller to set the metadata, and the seller is generally more interested in getting more people to look at their item then being correct. If a seller wants greater coverage, it's beneficial to deliberately mis-list their item so more searches return the item. As long as the site hosting the listings doesn't aggressively police their listings, doing this will provide a net-benefit to them. On the other hand, it corrodes the overall utility of the e-commerce site, but that's hard to measure, particularly if the site has had a laissez faire attitude to listing accuracy so far. I mean, I assume (admittedly a guess) that ebay/amazon would do better if they required accurate listings, but to get from the way it is to that would require a LOT of investment and moderation, and that would be a very hard sell.