With kids what I'm finding is that parents want their kids to "do everything". A lot of times their coming to class after music lessons, tennis lessons, youth sports, tutoring, dance class, etc, etc. Half of them are just stressed out because of all the activities and homework their doing. If their late it's usually because their parents have them doing something else before class, and then rush them to class.
With adults its other reasons. Some are coaching their kids youth sports, working overtime, taking night classes, going to parent-teacher nights.
Very few people and kids have the freedom to dedicate themselves completly to their martial arts training. Those that do will progress faster, and learn more. But those that still hang in there even though it's a lot easier to sit at home in front of the TV have value too. One way to look at it is that their fees are helping to keep the school open for the more dedicated students.
Simple fact of life in my school. Promotions are based on the "3 A's", Attitude, Ability, and Attendance. If your going to be late a lot, and miss classes, your not going to progress as fast as others. The choice is up to the students.