What is the security/copy protection model for streaming media services?

I realize this is a broad question, but maybe there is a simple answer

From my understanding, Streaming media services such as Amazon instant video, Netflix, and others do all decryption client side. What I do not understand is that if this is true, and the videos ever exist in a decrypted state, what prevents users from harvesting them en masse?

I am assuming the exact method of decryption somehow makes it less than trivial to just grab image / audio frames but I am wrong to assume that this is possible, or even easy?

I've always been curious how streaming streaming media providers circumvent this issue. I know some streaming providers (free ones like youtube) simply acknowledge that it is possible to save the videos, and don't do much to prevent it, but I would imagine that it is core to the business model of companies like netflix to make it neigh impossible to save a movie once you've rented it.

Answer

Just to make that this question doesn't seem too broad, I want to clarify that a direct answer to this question would show me how streaming media providers prevent users from trivially downloading their videos (via obfuscation / cpu features perhaps?)