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.
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.
The law like anything else is a system, justfab is exploiting it for fun and profit.
I really hope you are aware of the difference.