>> They could stop adult content from being shown to minors; it would just take effort on their part to do so

If you voluntarily sensor content, you might be in danger of being held responsible for various things since you control what people see. Phone companies in the US are "common carriers" which means they just connect people, but are not responsible for what people do over the phone (plotting crime or whatever). Social Media is still trying to have it both ways - censor some stuff but not be responsible for anything. IMHO that will eventually fail.