It's important to keep in mind that Tesla (like Walmart, Whole Foods, and Amazon) has been targeted by big labor for infiltration and union formation [1] [2]
This is not necessarily because working conditions make workers feel that they would prefer the tradeoffs of unionization, but simply because Tesla is much more competitive without its workforce unionized, and so labor groups that are entrenched in legacy car manufacturing wish to prevent Tesla from accelerating those companies' decline.
In many states labor unions have significant power granted by favorable laws that far exceed the market power they derive simply from organizing labor and leveling the playing field a bit. [3] [4]
So while I strongly support organized labor in abstract [5], it's difficult to blame Tesla for wishing to make its workforce less vulnerable to attack from big labor. Like many interest groups, big labor is mostly concerned with its own survival [6] and growth, and is not really about the workers at all.
For example, when Borders Books employees went on strike, it was the big labor groups that sponsored ruffians to join the employees on the picket line and make the strike much more aggressive. Similarly, whenever you see an inflatable rat alongside striking workers, chances are that rat was brought in by big labor to help dramatize the strike effort. [7] They also help name and shame fellow employees who aren't enthusiastic about the union to exert social pressure to make it form.
1. https://groundswell.org/whole-foods-employees-move-to-unioni...
2. http://www.sfgate.com/business/article/Union-Organizers-Targ...
3. https://en.wikipedia.org/wiki/United_States_labor_law
4. http://www.businessinsider.com/tesla-charged-uaw-employees-s...
5. While labor laws came to exist for good reason and helped improve living standards quite a bit, they went too far and tried to address too many issues via labor laws. Many things that are given to workers via labor laws should be given to all citizens, whether employed or not. Labor laws should not create an incentive for firms to eliminate unions, and should not act to correct social inequality. If such measure are needed they should happen via other laws.
6. https://www.unionfacts.com/employee/Air_Line_Pilots/TIMOTHY/...
7. http://mentalfloss.com/article/28466/story-behind-giant-infl...