Just because WalMart is the devil doesn't mean their wages should be regulated any differently than any other employers. Obviously a small business owner is going to have a harder time paying their employees more, but it shouldn't matter.
First off, there may be only a small percentage of people working for minimum wage, but there is a much larger percentage of people working for slightly above minimum wage. If the starting point is (let's say) $7.25, then $10/hr sounds like a lot, right? But it's still next to nothing. The minimum wage is still a guide by which employers will decide to pay their "non-minimum wage" employees. If the minimum wage was $10 instead of $7.25, then the people that are currently making $10, would be making (i dunno) $12... and so on and so forth.
Which is why we should raise the universal minimum wage, treating everyone fairly while trying to bring everyone to a living wage.