"Lawful intercept" backdoors like the one you described have been abused in countless cases (google it, you'll see - the example that comes to mind is the Greek

"Lawful intercept" backdoors like the one you described have been abused in countless cases (google it, you'll see - the example that comes to mind is the Greek olympics scandal). People are fighting this because the existing system is rife with potential for abuse, and anything else that opens up more potential for abuse/intimidation/corruption is a bad idea.