On the other hand, I'm in Sweden now. My phone bill for the whole year has been below $100 (I don't phone much, but I use internet a lot). So, "pre-paid" can mean a lot of different things.
And for those who don't really have a need for cell phones beyond the basics, they have an offer at 2 euros a month for 1 hour of voice time and 60 SMS. Even better, it's free as in beer (0 euros) if you have chosen Free as your ISP. http://www.free.fr/adsl/index.html
You'd really have to be bad at math to chose an expensive contract nowadays over paying in full for your own phone and getting that kind of plan.
Either way it doesn't make sense to charge $0 for a phone to consume resources by having a constant connection to cell phone towers and reserving a number. $7 is pretty cheap so I hope it's a fair price compared to actual costs.
That said, I can't imagine the carriers being too happy about having clients like me, who only charge the phone with $10 every three months ;)
There are typically two phone-minute charging models: "Calling party pays" (Eurozone, others) and "Both sides pay" (US, Australia(?)).
In a "both sides pay" scenario, each customer pays for their own side of a call. I call your mobile from my mobile, we're each charged a minute per minute.
In a "calling party pays" scenario, the person who calls pays for both sides of the call. You can tell when this is happening by whether there are different rates for calling landlines vs. mobile phones.
The important thing here is what this means for pre-paid services.
In a "calling party pays" situation, a mobile subscriber can continue to make money for the telco _just by leaving their phone turned on_, regardless of whether they can make outbound calls. The inbound caller will still be giving money to the pre-paid caller's telco.
With "both sides pay", the telco of the pre-paid customer receives all revenue up-front; there's no subsequent money to be made because all charges come out of the credit balance of the pre-paid customer. Between that, and revenue recognition rules, it makes sense to expire that credit every 1-2 months because there's no other way to get a revenue stream off that customer.