The union thing is standard for M&A, and isn't restricted to unionization efforts. If you begin the process of an acquisition, you can't suddenly fundamentally modify the asset being acquired with something seismic like that. It's the same reason the deal would become null and void if, say, this summer ABK announced an exclusivity deal for Call of Duty with PlayStation for the next 10 years. Or if ABK sold off all their valuable IP tomorrow.
If pressed, I suspect you'll hear Microsoft say something along the lines of wanting to talk to the workers themselves once the acquisition is complete, to discuss their work/pay situation and what their goals are, without explicitly saying they do or don't support the unionization effort. This lets them kick the can down the road on staking a public position, where they'll probably quietly hope the effort loses energy.