They might be restricting it to certain devices only, e.g. smaller phones and not larger tablets, unless you make a payment. What it's supposed to be and what it actually is, might be two different things. Do they have a list of supported(approved) devices for the free service? Could be they have a restriction of the maximum number of devices that can be authorised for the free service, like one, and if you want any more you have to pay. Basically to stop you sharing it with your friends and neighbours devices.
Some of these DRM'd subscription mobile services can be very restricted. My father has Sky Go, which allows you to view Sky TV on certain mobile devices within the UK. It works perfectly on his Samsung Tab, but when I tried it on my Chinese Oppo Find 7a, which is rooted and has a custom ROM, it didn't want to know, and I was in the UK at the time. Another thing with Sky Go, you can only stream to one device at a time with the standard subscription, not simultaneously for two devices, you weren't doing that were you? ...and so it kicked in with the payment option. Hulu is part owned by Fox, who are owned by Sky, so I should imagine they'll be doing the same sort of restrictions on it.