If I were a small non-union indie shop and I was lucky enough that some highly talented AAA developer wanted to work with me, I would be unable to hire him if he's union unless I converted my entire shop over to be union. If I didn't, he would be in direct violation of his union contract and could be fined or lose his membership.
I understand that not every union functions the same, but it is not hard to imagine that wherever there is a lot of money at stake the union will do everything in it's power, to well, seize power.
Considering the movie industry is dwarfed by the video game industry from a revenue point of view, I can very easily imagine this scenario repeating itself.
I've never heard of unions working like this, this may be a US-specific thing. Unions are just workers right groups, if the company is big, the group is inside the company, if it's too small, you just apply to the local union. They help to negotiate better laws and benefits for the whole industry & also directly at a company level in big companies.
The community theatre down the street doesn't require every performer to be an cardholding AEA member, and it wouldn't make financial sense for anyone involved (management, workers, or audiences) for that to change. The mere existence of a union doesn't necessarily change anything unless workers want it to change.
For big game dev companies that have repeatedly shown that they don't believe in treating people fairly, this would bring professional representation to the workers' side. Even if the details are light at this point, I don't see how you can be against that in principle. The employer/employee relationship is inherently asymmetric, and this attempts to correct for that.