My LQS that I'm about 1.5 miles away from does not have a toybox.
However, the quilting/textile goods store that's about.. 2-3 miles away has a lego table at the back of the store. I went in there with my oldest daughter about a month ago while my husband and my younger daughter waited in the car -she had fallen asleep- and told my daughter to stay at the table and play with the legos. I kept going to periodically check on her, and she was super quiet and playing by herself nicely. Then when I was ready to check out, I told her as soon as I was done paying for my stuff it was time to go. She actually was ok with that and I didn't have to fight with her to leave.
There's also a scrapbooking store near my old apartment on the other side of town that even had a little 4 by 4 area with toys too, and it even had a gate, so the kids couldn't take off and create havoc in the store. I think it's okay for kids to touch fabric -as long as their hands are clean. Before taking my girls into a fabric or craft store, I always wipe their hands and faces off with a baby wipe to make sure there's nothing that can stain or dirty stuff up.
I think it's a fantastic idea, and more stores should do it. Even if it's just a small box of toys in the classroom area -so it doesn't take up retail space- would be perfect!