Its a good will thing on both sides.
Some students call up (or don't call at all) at the last minute to say they won't be coming today because they have to go shopping (always on a CAVOK day). That leaves the instructor with a gap he can't fill and if its the only trip he turned up for a wasted day. If the penalty is enough to cover the instructors pay then good. I've never seen this enforced though.
We had one chap who did this regularly and he very soon found it hard to find any instructor that would take him.
On the other side of the coin if the school can't do the trip they should call the day before. However if the ac has just gone tech then theres little either side can do.