Isn't refusing life-saving medical treatment a form of suicide?

Suicide more the actual act that you self-inflict. By denying some treatment, you aren't committing suicide, you are just accepting your inevitable death. Many religions view suicide as a sin. Most religion is non-progressive, so the ideals of years past are still in use today. It's best not to look for logic, but to decide whether or not you agree with the belief system.