Freedom of movement is one of the essential freedoms (the others being trade, self-defense and speech). Freedom of movement prevents all kinds of tyranny, not only government-imposed, but also privately-imposed.
Concentration of power is only effective because it is concentrated. The US government only has power over the US. The New Hampshire General Court only has power over New Hampshire. Unitil only has a monopoly in Southern New Hampshire. By being able to move out of these areas, it limits the ability for the powerful bodies to exercise their power on any given individual.
This also explains why it’s damn near impossible for monopolies to form outside of government protection. Even if a store has 100% market share in a particular region, one can always use his freedom of movement to go and shop at another store. Therefore, it reduces any kind of monopoly (or monopsony) power a firm might have in normal (that is, no government interference) conditions. Especially, in this digital age, this movement is very easy. It needn’t even be physical. From my apartment, I can shop from anywhere in the world.
Wal-Mart’s ability to “exploit” their workers or “exploit” their customers is severely limited by the fact people can choose to patronize other locations. As long as people are free to move, then any claims of monopoly and monopsony in a free market are highly suspect and should be subject to extreme intellectual scrutiny.