If the digital (SD) feeds are being sent directly to Cablevision as digital feeds and simply being passed on then it may not be Cablevision's problem.
Someone in another form suggested that when HBO started it's HD feed they may have dropped support for DD 5.1 on the SD feed to save bandwith.
The HBO FAQ says, "Dolby Digital 5.1/AC3 stereo availability -- Can I receive Dolby Digital 5.1 audio on HBO/Cinemax?
HBO Dolby Digital 5.1 audio is available on our HBO Eastern and Pacific digital feeds.
HBO's commitment to Dolby Digital 5.1 is significant. 70% of titles, representing major theatrical premieres are shown in Dolby Digital 5.1."