I think it could be because some parents consider us to be "babysitters" just like teens who watch their kids when they go out and get paid for just that evening or day they are used. I don't think it is that they cannot grasp the concept...it is more that they do not want to because, if they look at it from our point of view, they have to accept that they should pay for the spot, regardless of how much they use it. I think it is human nature to always be looking for the "deal", wanting to pay the least dollar amount possible for the best quality.
I personally only charged for the days children were in my care but that was my choice. I started out doing that way and did not have the will to change it. If I did it all over again, I would probably charge for the spot because it was really stressful when there were weeks that children were sick and didn't come and I made way less than I had planned (seemed to go on for weeks
).