I've learned to look at the grain lines of the fabric before buying it.
Every once a while I forget - and sometimes want to smack myself.

I really do prefer the fabric to be torn at the point of purchase, for the very reason you have shown. It is beyond frustrating , especially when working with small cuts , and they are so off grain you have to purchase double to get the amount usable.

if your shop won't tare then check how it is wrapped on the bolt and ask that it be evened up before cutting.
This is especially true of wide backing. Cutting loses at least 8-12 inches. My local shop says they allow a bit of extra but it never compensates for the loss when squaring up the backing.
When I cut strips, I will tare the end and line it up before cutting. This can more easily be done after washing and then iron it straight.

I like tearing, one of the shops I go to tears the fabric.

That's a good idea. I'll try to remember that next time.

I agree with Holice that the worst problems are with the wide backings. They are folded and folded again on the bolts, which just magnifies the issue.

When we sell backings, we tear them, adding an extra 3" to compensate for the weakened edges.

Everything else, we cut. Carefully. There are a lot of quilters who would be horrified to get a piece of fabric that had been torn. (I think there are definite regional preferences with regards to this issue.)

I have that fabric.

Our stores used to tear all the time. I would rather they cut because when you tear cotton some of the threads will pull up and you have to cut that edge off to get rid of that, it is like a run in a stocking and I hate that look. If I have fabric that I think is too off grain I will tear to see what happens. Most quilt shop cotton is pretty good though.