The truth is.
The instructor is quite entitled to charge for supervising you while you are solo.
The reasoning, he has to stick around in order for that to work. Wether or not he has 1 or 10 students solo is trivial. He is still entitled to charge it.
Imagine if it's his day off, you ring up to ask if you can solo today. He drives down from X down to Y which takes him 1 hour and for that you are going to pay him nothing?
I can completely see the point there.
To sum it up, you are a student flying on an instructors licence. What you do wrong goes back to him and so he's fully entitled to receive his income.
I'm finding people love to get everything for nothing.
If you consider it simply monitoring, I assure you when things go wrong then it's no longer that simple.