It's not illegal; it's free speech. In their own business. It is incorrect, it is false propaganda, but it's free speech.

That said, Amazon especially has engaged in enough illegal or at very least highly highly questionable practices in attempts to avoided workers unionizing. Such as: they had an agreement with the city to speed up a stop light, so organizers working in the parking lot didn't have free time to effectively talk to workers leaving.

You can get in serious trouble, fired, for talking about unionizing at work in their workplace. Their rules. But the parking lot is fair game. So...they tilted the board to favor them, even there.