Lots of comments about this not being cost viable for the airlines to give up seats for these purchasers, but I read your remark a different way - the airlines will note which passengers purchase products on planes, and adjust where these passengers automatic seat selections put them in future - combining them with other data that they have on their customers (e.g. frequency of flights, if they're with children, etc) to try and maximise probability of purchases and influencing more people to purchase over time.

Anecdotally, the people sat next to me on my last flight purchased wine, which resulted in me getting a mini-bottle too. Doh!