As opposed to pretty much any other PAYG tariff? Where it's also buried deep. It says it's only free till the 1st October right on the page where it tells you all their prices:
http://giffgaff.com/index/offer
Seems clear to me.
Just looked:
O2, similar, but to get the costs out of "bundle" you need to head 3 clicks into T&Cs
Given up on finding them on vodafone, all i could find was a reference in the T&Cs to "pricing plan" but the price plan page didn't list data.
Orange are probably the best, one click through again, but they have a nice table.
So I'd argue lack of clarity is an issue across the industry. So, to be fair, I do concede your point having just argued it for you 😉