Alabama is a right-to-work state. My understanding is that it would be completely legal for Amazon to hire new employees the second someone doesn't show up because of a strike, so I'm not sure their leverage.
These workers are probably making between 30k and 50k a year so $500 isn't nothing.
In the example of something like missing a day of work, the contract will spell out the specifics of advance notice for days off/sick days. If it's completely unannounced, the contract will spell out the steps of a disciplinary process.
Source: I work in a right to work state, used to be a union member, asked this very same question at the time.
B) Not every state is a right-to-work state. So, from a 'worker's of the world unite' perspective then obviously making a union in Alabama will help other places form unions.
I could be very wrong and perhaps the union could get 1 or 2k more per year - but I am skeptical.