I agree with that and wish most players saw it that way
Most player who don't pre-register but expect to show up day and be allowed to register even if the field is full do so because they know from past experience that TDs will knuckle under and accommodate them. And they're not going to change their behavior until they're turned away.
A simple but firm, "I'm sorry you drove all the way here, but the field is full. You might be able to get in if some of the players who pre-registered don't show. I can put your name on the waiting list if you hang out a bit. Right now there are people ahead of you on the waiting list" is usually sufficient to get the point across, although some of the slower ones may need to clank off the chastity belt two or three time before they see the light.