Digital music sold used to have DRM, because it just couldn't be avoided since the licensing terms of the content required it. The Primecast seems likely to be the same.
It's more like them saying, "Sorry, user, we can't let you do that."
Where as this (and what Apple does) is more like, "What's that user? You want <competing thing>? Well, fuck off!"
Thinking on it though, massively slowing down deliveries of books from publishers Amazon doesn't like was actually the first time I've seen Amazon do something like this. So this actually isn't the first time; now its a pattern.
Side rant: Amazon Prime's media crap is also especially annoying to me personally, since I spend most of the year outside of the US, where it doesn't work at all. (Go Netflix!))
Now I don't know the licensing terms they have with the content providers, but they certainly aren't fulfilling the consumer demand.