Are you sure that's the case? Here in the US, Verizon and AT&T prompt you to set up a recurring subscription, but you can cancel it at any time.
In any case, it's a bit interesting how the carriers have priced the plans. You pay less for the 5GB plan ($35 vs our $50 - give or take the exchange rate which is about $1:$1 right now), but none of your carriers has a $20 1GB plan for light users (just 250-500MB for $15). I like your $35 plans a lot.
If Verizon would boost their $30 plan to 3GB (or even at $35), I might just set it and forget it, and use the iPad to tether, but for now I'll see if I can make do with the 1GB plan and tether only occasionally (at airports, etc.).