The democratic process is the democratic process and so I respect their decision but grad students unionizing is really more complex than the workforce. In particular, the union is turning over every 5-8 years. It is quite possible no one wants the union after those who voted it in and don't have a choice not to join. Your PhD is defined by your advisor and my #1 advice to future PhD students is to pick your school based on advisor before location and other things. What this means is that your choice to "join" the union is incredibly overshadowed by external forces.
Again, as someone who went through this very recently, the problem is the power imbalance between the institution and the students. They have complete control over your supply of labor. That is, many (all?) US universities usually have the policy "any outside job is considered incompatible with full time PhD work and grounds for removal from the program". To me, forcing universities to pay students their prevailing market rate would solve all these issues. The first step towards this would be a cultural shift that allows part time PhD and moving towards PhD students being employees and not students. This would require a massive change to the F1 visas to allow international students to work which really pinpoints the root cause of all of this. PhD students are really in this strange limbo of being neither a student nor an employee. This limbo allows the university to use whichever definition fits their prerogative just like the NCAA. For instance, PhD students want more pay but the university decreases tuition assistance, nothing has changed.