PlusMinus definition unexpectedly affects plotting


In version 10.1 I found a strange Plot behavior I did not anticipate. It seems that various heads including PlusMinus are treated as wrappers and stripped by Plot in a way that is not safe. This means that definitions that do not appear in the plot expression can radically and errantly affect its output:

ClearAll[PlusMinus] Plot[x^2, {x, -1, 1}] 

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PlusMinus = Power; Plot[x^2, {x, -1, 1}] 

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Surely this is a bug? What versions are affected? What is the easiest way to patch this in version 10.1?