I think we should keep asking that question about everything spoken or printed that someone, somewhere might potentially disagree with or find objectionable. Free speech means freedom from disagreement and objection. So all speech should be approved ("allowed") by the proper authorities before being able to be communicated. Free speech is restricted speech.
Same reason why Amazon can cut service to Parler. The First Amendment guarantees the right for private companies to govern their own properties, literature, and procedures.
If you do not like it you may criticize them publicly, boycott their business, or start your own competing employer with the policies you prefer.
I'm personally glad that Amazon are able to express this sentiment for all to see. That the only time they've ever supposedly cared about the spending power of the workers they employ concerns union membership dues should be quite telling. They're betting that unions are going to cost them money because they believe that high union membership will lead to better wages, costly improvements to working conditions or other costly responsibilities. There's no other conceivable reason for them to publish this trite because it is in their interest to pay as little as possible for as much productive power as possible. It's not at all in their interest that you could buy a fancy dinner for the money you'd spend on membership dues.