There are a few reasons. First of all, UMD is part of the ACC. The ACC runs itself like a corporation. You have a quorum, and minimum number of necessary votes, annual meetings, etc. The schools agree that all bylaws or ratifications will be met by a supermajority. In this case, the proposition to raise the amount was agreed to. UMD as a member of the ACC is forced to follow whatever the majority issues. UMD cannot just pick and choose which rules it wishes to follow, just like any stock holder who loses out on a company vote cant just still choose to disobey that law. And to a similar extent, lets say I vote for the right to bear automatic weapons and it doesn't pass.. I cant just disobey that and start carrying automatic weapons under the pretense that I voted for it, nor is how I voted evidence that I broke the law any less. Simply a rule is a rule, once it becomes a rule. If UMD doesn't like the rule then they can take LEGAL ways to get out (See: Next Paragraph):
Sure, the ACC is more of a "club" than our democratic democracy in the 'automatic-weapons' example. But, UMD then could have also made the case immediately after the increase was ratified. Maryland could have plead to a court saying i need to get out of the ACC because I can't afford this fee or for whatever reason. They could have LEGALLY tried to break their contract with the ACC. Instead, they continued accepting all the benefits of the ACC. The continued to receive ACC money and continued to brand themselves as an ACC member. They accepted the $50 million bylaw and are therefore bound by it (in an ideal world.
With that being said, this is going to court. UMD is going to argue that their contract with the ACC shouldnt be enforceable, that specific $50 million clause isnt enforceable, and the hailmary, even if it is enforceable $50 million is not reasonable. With that being said, what is their vote evidence of IN COURT. It's not evidence of anything. They tacitly accepted the new buyout, whether they agreed to it or not.
I can tell you however, from extensive experience, Courts almost always tend to favor the government, municipality, school, whathaveyou, that has to PAY the money. Courts do not like public universities wasting public funds. They tend to side with the UMDs of the world in these scenarios merely because our Courts want to save tax payer money. Even if the Courts disagree with UMD they are still likely to do something to appease ALL parties, including UMD.