[SDGT]

It's the steam model.

"Why not buy this game if it's only $5?"

20 minutes, 10 purchases, and $50 later with another set of titles to stale in the library.

[oddevan]

But there's also the iOS model to consider:

"Why buy this game if it's not $1?"

I know there's other factors at work, but this is what publishers are (understandably) afraid of.