I agree it's unethical. This is a commercial site, it tries to offer goods in exchange for money, anything that costs money should be emphasized and explained. How would they feel if someone managed to take some of their merchandise by stealth?
I don't think this is some grey area: if you tricked me into paying a sum I never chose to pay, you're stealing money off my pocket.
That reminds me of certain ads I sometimes see on the french TV for ringtones or silly phone games: if you read the small print you realize they actually offer a membership, not a product, and you actually pay something like 3 euros a week instead of a one-time fee. They call that "clubs" or something.
Those ads usually seem to target teenagers (they're mostly on music channels or channels aimed at kids) who probably aren't in charge of the bill and if they're not careful they might end up with a bad surprise. This is beyond shoddy.