Why does the grocery store have an express checkout with the sign that says ‘limit 12 per customer’.

One hypothesis is that it is in order to make sure the line remains fast, and people could quickly leave the store. But, if you notice, the express line is always the same speed as the other lines. There are more people at the line because it is ‘express’ and each individual coming to line up attempts to optimize for time anyway so the lines are roughly take the same line to get to the front.

Limit 12. So, what, 14 and they don’t take you? 8 and they do? Should you get 14 and go to the other line or get 8 and go to the express line?

As soon as you are thinking this the sign has worked. 8 items! But before you were probably just coming to get milk and bread. You have been primed.

The true purpose of the sign reveals itself.

This is just an example of how suggesting that you make a decision can lead you do doing things that you would not usually do and is an excellent example of how ‘choice’ is different in kind from freedom.