The better a store manager controls expenses(such as labor) the bigger their bonus. Having a union is a check and balance against this type of favoritism.
Quite often the more experienced person is a cost saver over using cheaper labor. I certainly charge more for my services that I did when I was inexperienced.
What you describe as being favoritism is completely foreign from my own understanding of the term, and I'd guess it isn't what most people would consider favoritism. I suspect that we have a fundamental disagreement that won't be resolved by my picking further at your word usage.
I do believe unions are necessary, but if I were convinced that this kind of thing was really the reason why unions exist, I'd have to reconsider that position.