That is true that the girls do have a choice, but they are never given all of the information. Kyubey targets young people, who may not have the emotional maturity to consider the weight of such a contract. As for those who do, Kyubey dodged questions and rewords things to make the contract more appealing. The girls “sign up” for a magical wish and an adventure of Witch-hunting; they don’t sign up for turning into witches, watching other magical girls die, their souls isolated from their bodies, negative emotions literally killing them or having to enter Lovecraftian hellscapes in order to murder enemies they know nothing about. But because they “consented” to this and they “chose” this, they’re expected to be grateful. In the end, it doesn’t matter what people sign up for. People should expect and deserve to be treated fairly by a contract.

Capitalism can be a lot like that; jobs that are advertised may have poor working conditions, abusive employers, workplace harassment, working hours which drive people into exhaustion and depression etc. But people sign up because of the idea that they can make money, make their lives and others’ lives better and make their mark on the world. And, like the magical girls, due to people’s living situations, those who are particularly vulnerable may not necessarily have the option to quit. Again, this is only one interpretation of the series. I may not be explaining it properly, but there’s an amazing video essay about it that explains it way better than I do; https://youtu.be/9OVFifBE3vA