By creating an environment of respect in employment terms. In a word, inertia. There is a reason employment contracts with greater than 40 hours a week are almost unheard of. Union blood was spilled to bring it down to that.
It's unreasonable, (and depending on the jurisdiction, illegal), for corporations to make a difference between unionised and un-unionised workers. So by promoting improvements for their members, they promote improvements for all employees.
Why is it unreasonable? There are plenty of auto manufacturers in right to work states who do just that. Why shouldn't anyone be allowed to work for Tesla in Buffalo regardless if they want to be a part of a union or not?