Shear stress on a bar

I'm trying to understand a problem found on Mechanics of Materials and did not completely understood the problem. I can solve problems like this but I still don't know why I'm solving this way.

It is a simple question: Why the shearing force V does not cause shear stress ($\tau_2$) on the point A as it does on point B? (page 575 pictures c and d for reference)

Yes, I've searched on the book but did not find why

Thanks

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