This a common practice and I actually wish all fabric stores did this. I used to work at House of Fabrics and when we did fabrics for quilters, we had to rip it. When you rip it, it tears straight down the weave of the fabric and is much more accurate than cutting. I know it it's a little messy, but it's better to deal with the strings than getting home only to find out that your cut was way off and you don't have enough fabric for your project which seems to happen all too frequently.