Virgin is an interesting airline and tends to do interesting things like this. A few years ago, they touted that they had higher prices, but also had the best coach experience out there. It failed miserably. When booking flights, it turns out that pretty much the only thing that people care about is price.
A BOGO sale is right along the lines of something you’d expect to see from Virgin and personally, it’s really nice to see an airline trying to shake things upz a bit.