I don't think they'd be required to but I don't see how they wouldn't have.
Definitely not required, but the coach would look like king of the A-holes if he didn't honor the scholarship.
There are some clueless coaches out there though, so maybe it has happened. Probably pre-Internet, though, as the bad press would be overwhelming these days.
Don't forget we are taking about Scott Drew who is the king of the a-holes based on stories of his recruiting tactics.