In this day and age, having any download limit or throttling would be a handicap to a very large section of users i would of thought. If you think about it for a few mins, a lot of peeps that are buying new TV's will be buying 4K tv's. Most content for a 4k tv is streamed, even if it's through SkyQ. One 4k film, even a compressed film via SkyQ, will be 25GB+. Try downloading that at night when your being throttled by your ISP and you will get annoyed very quickly. Try doing it twice a month when your monthly limit is 50GB and you will get even more annoyed. At the moment i'm only just over a 2 weeks into the current month and have used 50.3GB and i'm a light user.
As you said, Zen don't traffic manage and are unlimited. Perfect for peeps that want to take advantage of streaming and using the net as it is supposed to be.