It is a logical extension from the current "no junk food" mantra public schools are going on about.
Once you accept that diet can be controlled, or any minor thing can be controlled, then you accept that everything can be controlled.
Once you allow...let's say...the government, any government, saying you can regulate what you can see on television, you logically must then accept the government regulating all aspects of your life and if something is regulating you, they need to watch what they control.
There is a reason it's called a "slippery slope". It's because once you accept one premise you must then accept all following premises to remain logically consistent.