This is extremely threatening to those who wish for capitalism to be a zero-sum game.
In order to keep prices low Wal-Mart is a very aggressive buyer, meaning they squeeze their suppliers and look for the best sources. If the buyers don't like it they don't have to sell to them but because of the volume they do. That is the same reason many companies go to China.
Wal-Mart is also probably the leader in diversity hiring. I have seen many old and handicapped workers who I doubt are worth what they are being paid even if it is minimum wage.
The left hates Wal-Mart because they are the epitome of free enterprise. The unions are just an arm of the Communists Party, as is the Democrat Party, so they have a common enemy in the free enterprise leader of the world. They are all on the attack.