"inappropriate and undue influence by the [National Labor Relations Board]"
From the NLRB's website we can see the kind of thing Amazon deems inappropriate:
The National Labor Relations Board is an independent federal agency that protects the rights of private sector employees to join together, with or without a union, to improve their wages and working conditions.
Federal law gives you the right to form, join or assist a union; choose representatives to bargain with your employer on your behalf; act together with other employees for your benefit and protection; and choose not to engage in any of these protected activities.
The law we enforce gives employees the right to act together to try to improve their pay and working conditions, with or without a union.
The National Labor Relations Act forbids employers from interfering with, restraining, or coercing employees in the exercise of their rights.
So with its latest outburst Amazon has still not disabused anyone of the notion that it is a poisonous viper's nest.