Apart from the issues of breaking contracts or the head count now being wrong it seems to me there is a practical issue.
If the airline doesn't sell A-B tickets, only A-C, will anybody be allowed off at B? Unless they need to offload everyone for some reason (fuelling?). If they don't let anyone off you'll stand out like a sore thumb.
[History story]
I read an article many, many years ago about a Delta flight from Washington to Atlanta. It stopped in Charlotte and Delta couldn't work out why people were getting off in Charlotte when they had tickets to Atlanta. Then they twigged - the fare was cheaper to Atlanta than Charlotte.