IMO the key difference is that you were getting mail from political parties. I think the correct comparison would be if the US government were sending you mail

IMO the key difference is that you were getting mail from political parties. I think the correct comparison would be if the US government were sending you mail instructing you on how to vote for the party already in power. There's a big difference between the party pleading its case to you and the government itself endorsing that party. I think Amazon's power over employees makes it more like the latter, although neither is a perfect analogy. If I had gotten mailers last year directly from the United States government instructing me "how to re-elect Donald J. Trump", I would have been livid. Individual within the company should be able to make their case for why they think unionization is bad, but I don't think the company itself gets a say in this.