For example, at many companies you only accrue vacation time if you work full time. However, since there is no actual law regarding vacation time (a company doesn't have to give you any if they don't want) it doesn't actually matter what they consider full time or part time for that. Some companies called 40 hours full time. I work 37.5 and I am considered full time and get some great benefits for entry level in the US.

The only other thing besides the new health law that determines "full time" is overtime. You get paid time and a half if you work more than 40 hours, so I would say as a general rule of thumb 40 hours is used to determine full time vs part time, but not everywhere and not all the time.

With the Affordable Care Act it's moving to 30 hours for healthcare benefits. For many things it doesn't actually matter and "full time" vs "part time" is just made up.

In addition to all of the issues with your math that everyone else has outlined, why is every single person at Walmart getting a raise? Many people at the corporate level are probably doing just fine, it's only the retail workers that are getting really shafted. So if not everyone needs a pay hike you can give a larger pay hike to the people who really need it at the bottom.