I am not at all certain that I agree with this statement, particularly since you are posting this with other questions about civil liberties and the First Amendment. I will explain why I disagree, but then I will discuss why someone might claim that this is so.
Crowd control and thought control are not particularly closely related for the simple reason that controlling a crowd can be done regardless of what that crowd is thinking. Let us say that you have three crowds. One is protesting for abortion rights, a second against abortion rights, while the third is coming out of a football stadium. There is a need to control all of these crowds. All of them could become unruly and lead to lawlessness. By controlling the crowds and keeping them from breaking laws, the police are not making any statements about the crowds’ ideas nor are they trying to mold the crowds’ thoughts in any way.
If I had to claim that the line really is “very thin indeed,” I would first look at the textbook. Since this question comes from the textbook, the text must surely say something to defend it. Outside of that, I would argue that the actions that a crowd takes to express their views are nearly the same thing as the views themselves. Imagine, again, that you have an antiabortion crowd. Let us say that the crowd wants to express the depth of their revulsion for the practice of abortion by marching down the middle of a major street and blocking traffic. If the police prevent them from doing so, they are preventing the crowd from fully expressing its opinions. If you manipulate the ways in which people are allowed to express their opinions, you make it harder for them to hold those opinions in the first place. Thus, controlling crowds and making them act in certain ways is very close (one could argue) to controlling, or at least manipulating, the ways in which they are thinking.