Hm. In my opinion that isn't as bad, but I can't put my finger on why.

This is one reason: https://news.ycombinator.com/item?id=26263866

I thought it was weird, but I can't imagine a basis for not allowing that.

Even if it was someone's boss... they too get to talk / have an opinion... even if I think it is wrong. I've certainly worked for employers who I thought had some very wrong ideas, I still did my own thing voting wise.

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.