What, exactly, is unfair about the current compensation? Walmart pays exactly what they and the employee agreed to at the onset of the employment agreement. Those who couldn't reach an agreement have do not provide work for Walmart at all.
I really don't know how you can get any more fair than that? Everyone equally has the ability to decide what works for them and are free to move on when it doesn't work.
> If workers have room to negotiate
A union actually takes away a worker's room to negotiate. The union becomes the only vendor and, in this case, Walmart would be legally required to hire through that vendor for the defined positions. This is less fair, as now you are managing the supply of labour and actively hindering competition. A union has no bargaining power if it has competition, of course.
While the benefits of a union are clear, the way you try to spin the situation is, to say the least, interesting. A union is beneficial because it takes away the fairness and takes away the freedom to negotiate. Fairness and freedom are what allow people to drive value towards zero, which is where the problems stem from.
I have a feeling this will be taken as an anti-union message, even though it really is a pro-union one. The loss of fairness is a good thing! We have a long history to look back on to see why.