Many years ago, I used to work for Vodafone - and had some responsibility for their filtering stuff.

One of the main problems was that, at the time, it was all managed by Blue Coat. As they were a US company, many of the "violations" that they picked up just weren't considered problematic for a UK / EU audience.

Similarly, they would use blanked keyword blocking. So sex education resources would frequently get blocked. Although, again, that may have been by puritanical design.

Anything which looked like it might be used to bypass their filters (VPN, proxies, etc) were also blocked.

Ultimately, the customer isn't the user. It is the network who are terrified that Johnny's parent is going to go to the papers screaming about how the evil phone company corrupted their innocent child. That's all it is there for - to protect the network.