I think what NowTV offers is quite generous, but you have to remember that Sky wants NowTV to complement Sky, not cannibalise it, by bringing in extra revenue from people who would never sign up for Sky, and not by losing revenue from people who realise that NowTV, plus Freeview, gives them everything they were getting from Sky. Which, if you could record on NowTV, would suddenly become a very much larger slice of Sky users.
So Sky keeps the recording ability to itself, augmented by not allowing On Demand on NowTV Sports.
Beyond that, what NowTV users are allowed to do is finely judged by Sky; I get the feeling they will have agonised about keeping Sky Atlantic reserved to Sky, or being on NowTV like it is, and I am very glad that the spreadsheet of pros and cons must have gone our way.
But if you think it’s crazy that you can’t record, then you are one of the people that Sky does not want to lose to NowTV; and the remedy is in your hands. So actually, Sky would be crazy to let you record on NowTV....
You can now record NowTV on YouView boxes if you subscribe to NowTV via BT TV, having taken BT as your ISP, and record from the NowTV channels that BT TV provide.
But outside of that, good luck finding any legal device that will record from HDMI, never mind what Sky might think; but such a device would not be a digitiser, as the HDMI signal is already digital.
However, you could make a recording, even an HD one, via the ‘analogue hole’; use an HDMI to component video converter, which you can readily buy, and an old analogue HDD/DVD recorder, which you might find on eBay, which lets you record from the AV inputs.
Might even support timed recordings, you never know.
Last I looked a Black Magic design H.264 pro recorder was legal, while a cheap and cheerful HDMI splitter will filter out the copy protection, possibly by accident, because it's cheap! Injecting an LTC time code to the signal can be tricky. All in all it is a bit of a performance to set it all up, and hardly worth the effort except in exceptional circumstances!