I think the difference is that defeating the other team is the point of sports, whereas at least ostensibly the law is supposed to provide a set of coherent rules for businesses to compete against each other. Trademarks are defined according to certain legal rules, and if you have one, this is how they provide you with a limited monopoly in a certain context. Allowing businesses to define property law through contracts lets people define the rules however they want. And that leads to irrational results.