EE do discounts on large data bundles quite a lot. I think with a network that has quite a lot of capacity now, they're quite keen to get you spending more money - in return for data that's probably fairly cheap for them to provide.
Even if this offer has ended, something else will be along soon.
I wonder if this is the future of EE under BT, where you now have quite expensive plans to enable the BT-style offers that are almost never ending. The key is of course to make sure you're signing up for a discounted plan, rather than a time-limited discount (e.g. first three months half price). But that should be quite clear.