In a litigation free world, I would agree. Let him sink or swim and maybe he will realize that he needs that pre-sparring workout to develop technique.
BUT there is the safety and liability issue:
1. The pre sparring work out also acts as a warm up, physically and mentally. If he doesn't participate he might not be properly warmed up and prone to injury. Plus, the pre-sparring training gives the instructor the opportunity to get a feel for each students mental and physical state and personallity before he cuts them loose on each other with hate and discontent. If this guy is harboring an attitude or is a jerk, the pre sparring stuff will let the instructor see it so he can either refuse to let the guy participate or take him to task and reorient the student on the rules of the road in class.
2. Because of the above, regardless of the waivers, contracts or anything else, the instructor could be held liable for any injury to the student or collaterally liable for any damage the student does. If the instructor strays too far from the program that he normally adheres to and only teaches this student a part of it, it could be considered unethical business practice.
Sort of like a teacher who normally is preparing students for college exams and only teaches the fun "Jerk it to Moron as Blank it to blank" section without assessing the total skill/need of the student. It is unethical to let a student engage in sparring or dangerous activities without assessing/training the student to a level of readiness.