Virgin America don’t make it clear that you don’t have to select a seat. Instead they show you all the available seats on the plane, along with prices for each.
It’s not obvious that this is an optional cost, you’re not obliged to choose a seat. At the top of the page is a ‘Skip’ link but it isn’t made clear that this is the free option. The use of the word ‘Skip’ could lead some users to think they’re only temporarily deferring seat selection. Nowhere is the word ‘free’ used.