Well, I did wonder about that as well! What concerns me as well is that they can apply charges to your mobile phone without any form of agreement from you.
I know this happens with direct debits as well as my wife had problems when we initially signed up to a telecomms provider but then changed our mind and cancelled well before the start date. She then got a direct debit against her bank account and, when she phoned the bank to query it, she was told that companies don't have to have written proof of the customers agreement. Anyway they refunded the money and we thought no more of it until the next month when there was another deduction from her account. She rang again and asked why they had allowed another deduction despite having been told that my wife didn't want this to happen and go the reply that "it was more than 16 days before the second deduction was made"
I also had a similar problem when, two years after having left a railway modelling group, i saw that a direct debit had been taken from my account. i was told that basically it was my fault because I hadn't blocked the company from taking money from my account. I had a lot of problems getting the money back from the group.