People have the tendency to go beyond invisible boundaries. Most do not pay attention and common sense if often uncommon. As long as it does not say that it is not forbidden then people will assume it is ok. Often, that is the ready justification. Aside from that there are really people who are simply hardheaded and will go even when there are portable barriers already set. How much more if there are none? Falling in line, especially when it tends to go snaky, is best with such help. It also prevents others who sneak in to get ahead of the line to do such.