The point is that you pay for the service for a certain period of time at a certain rate. Once that period has expired, you can choose to sign up again at whatever the current rate is, or you can choose not to sign up. I don't see how anyone is locked into anything.
As with majority of subscriptions you have to opt out to get out - this is the case for GP where you continue to pay unless you cancel the sub. Which a lot of people neglect to do.
Granted some companies make this more of an hassle. Take cellphone companies for example. A lot of people also subscribe to so much stuff forget what subs they are paying for, which is what these companies want.