Nothing. Just like nothing stops the ones that do want a collection agreement as they work for Tesla and have join the union. There's no requirement for a majority wanting it or not.
It's not a question of any majority. If you don't want to work for Elon then... just don't. There are many other employers. He can't force you to work for $2/hour regardless of whether or not there is any collective bargaining agreement when you could get more than that across the street.
They are employees, members of a union, so the union has the right to demand a collective agreement. Tesla has the right to not sign it, but we have the right to strike enshrined in our constitution and sympathy strikes are legal.
* Is there any place for government to have labor laws at all? Because that argument applies to everything from pay, nondiscrimination, and safety.
* Do you not see unions as a market force? Government generally does more to hinder unions than support them and the laws on the books in most countries are there so naive owners who didn't read their history don't do go and get themselves dragged out of their office by an angry mob.
What stops them from quitting at any time, or not working for him to begin with?