"Why are there so many Mexicans (percentage-wise) shopping in Wal-Mart, while other places have no where the same proportion shopping in their stores?" Well, I think a case could be made that everybody pretty much shops at Walmart, therefore in areas where there are a lot of Mexicans living, there are bound to be a lot of them shopping in Walmart.
No, I said, percentage-wise. I can go into Target or Kroger and hardly see any Mexicans.