Because the law is a reflection of what society finds acceptable, interacting with what an individual's rights are.
Society finds medical care, dental care, acceptable, and an individual's rights are rarely impinged upon by having such work done. That's a no brainer. No one's rights are infringed upon by having medical work done. And if and when they ARE, they do have a right, regardless of the rulings of this case, to bring their case before a court. Cases where consent was not given have come before courts before, especially in suicide cases, where clearly the intent was to die. Society has taken a stand and said that they don't approve of suicide, and so medical professionals can continue regardless. It's always a balance when it comes to these issues.
in this case, the SCoC has said, essentially, that the right to security of person is something that we can't give away, and that the situation she was in was one which violated that basic human right. Comparing it to medical care, where security of person is planned for, strived for, at every turn, is a flawed argument at the very best. The two do not compare in the end, and people can take it to the courts all they want... the court will continue its balancing act, and medical and dental care will continue just as they always have.
