It's absolutely licensing.
When content is licensed, there are different licenses that are distributed based on who pays and what the service is:
* First-run -- you get to broadcast it first
* Syndication -- you get to broadcast second
* Digital -- you can also show the product on a digital platform (this was how Netflix was able to do their loophole with Starz back in the day, Starz sold the digital license and streaming license to Netflix because the content owners hadn't specified that such a license was unable to be resold or relicensed -- that' since changed and now it's an extra fee if you want to do that like EPIX does)
* Digital streaming -- can be streamed but not watched offline
* Rental -- traditionally DVD/Blu-ray
* Digital rental -- iTunes, Vudu, Amazon, etc. -- includes offline
Netflix would have to pay significantly more for its content to offer an offline mode. Even HBO, who owns most of its original content -- doesn't do offline because again, the costs would be exorbitant.
EDIT: To be clear I'm not being all-inclusive, as JFB points out below me, there are even nuttier licensing schemes too. And this is before we even talk about international, where you typically have different agreements with every. single. territory.