Here is the thing about Amazon ... they are so large and the company so well positioned that IF they vote for unionization then Amazon could just as well close the facility completely and move it to another state where they don't have to contend with Unions. Given it's size an finances the elimination of a huge building like that and moving it to another location would be a no brainer for them and wouldn't hurt them one bit. The result to the local economy and those workers who would then be unemployed would be far more consequential.
I do believe that there are cases for a union and our history is replete with them such as the Railroad industry and especially mining but there has to be a give and take situation in every case. I do believe that Amazon buffers on a modern day need for unionization but given their size, unless the government got involved, Amazon would just as soon close up shop in Alabama and move to another location and do so for creating an example only.