The Coase Theorem essentially states that if trade in an externality is possible and there are no transaction costs, bargaining will lead to an efficient outcome regardless of the initial allocation of property rights.
But a student being informed about risks should not be just cause for a complete liability waiver for a university or the NCAA. This type of clause is best designed for adults of relatively equal bargaining power in a business transaction or at least where there is an employer-employee relationship.
Whereas in a purchase, the deal is through a corporation which removes the personal nature of the transaction so society gives you complete freedom of speech to discuss your bargaining skills.