The reason: It's been a long-standing myth and truly a bunch of B.S. Of course, most people have bought into this myth. Bottomline: If you are hungry, eat! When you are eating a healthy diet, there's no need to worry about eating ... at any time.
Most likely the reason for this rule: After dinner snacks usually consist of cookies, cake, or ice cream. That ain't healthy eating.
Source: Eat This, Not That Tweet