@boomzilla I think it has more to do with the "similar things" strategy. It works fairly well for books (if you like one genre, it makes sense to suggest choices of the same genre) and this is where Amazon started, so I guess this is why we get the same strategy for other things.
Then there is the fact that some items are sold by many different sellers, so since these recommendations are based on "other people that looked at X also looked at...", obviously when I buy a FooGizmo I compare a few sellers of FooGizmo to check prices (including shipping) and stuff. Again, Amazon probably didn't account for that when they built their recommendation strategy, since they were initially the sole vendors (and wouldn't usually offer the same book more than once, since stuff like hardback/softcover are options for one book, not different items).
So I'm not sure there is really a defined strategy behind this, it's probably that it works "well enough" in a lot of cases, there is very little cost to them to getting it wrong (at worst I have a chuckle seeing some weird association, but it's not like I'm going to cancel a purchase for that), and this is what they already had.