Eh, there are lots of purists out there who have their idea of what's "right" and what's not. Really quilting is an artform, and there is no right or wrong. There are still hand quilters out there who look down on machine quilters - and how many of us hand piece and hand quilt our quilts these days? Some do but the majority of us indulge in our machines, and without guilt!
There are lots of ways to bind a quilt, and turning the backing fabric over to the front is just another method. Nothing wrong with that at all, and hey, maybe Mom didn't mean it to sound as harsh as it did. We all say things sometimes without realizing how it might affect others around us, especially those close to us because we tend to let down our guard and be less careful about what we say because we are so comfortable. I bet if she knew the feelings it brought up in you she would have taken it back.