It's not necessarily so. However, what I have noticed about Walmart fabric is that often the "hand" is not very nice. The fabric is often stiffer and not as closely woven as quilt shop fabrics. Also, the patterns are often very slightly misprinted and the colors are often not exactly right.
Theoretically, at least, quilt shop fabrics -- even if the pattern is the same -- are made out of a better "griege" with more attention given to exact colors and exact pattern placements (so you don't get a slightly smudgey look from one color not having been aligned perfectly with existing colors). They are also supposed to have been given additional chemical baths or treatments to soften the "hand" of the fabric and to protect against fading from sunlight (although that protection is gone once the fabric is washed).
However, buying from a quilt shop is not a guarantee of getting higher quality fabric, and sometimes Walmart is able to purchase lots of higher quality fabrics at a huge discount. I have seen some of the same fabrics in both places where the difference in quality was pretty obvious, but I have also seen the same fabrics in both places where it seemed as if the Walmart fabric was exactly the same quality as the quilt shop fabric that cost two or three times as much.
In general, though, I have found that the patterns in Walmart fabric are not nearly as sophisticated as those I find in quilt stores, and that the hand of Walmart fabrics is usually not as fine. If you practice looking closely at the fabrics and touching them, you learn to see the differences pretty quickly. I wouldn't hesitate to buy a Walmart fabric, though, if it met my criteria for color, pattern and feel.