It doesn't, but contract wording can be an issue. HBO's contract with Universal and Fox may say something along the lines that "HBO has exclusive streaming and over the air download on these titles". iCloud, while not streaming per se, could be viewed as a violation of that contract. However, as you said, HBO has nothing to loose by reworking the contract to allow for iCloud and similar services.
Also, in response to some of the other conversations, I never once got HBO, but if they gave me the option to download an app and subscribe to them directly (not going through a cable company) I would hop on it! I've already dropped cable for Netflix Streaming, Hulu Plus and iTunes. Add HBO Go and I would be a happy panda!