‘Canvas image extraction prompt’ displays useless message

From 0021-Add-canvas-image-extraction-prompt.patch:

+canvas.siteprompt=This website (%S) attempted to access image data on a canvas.  Blank (white) image data was returned this time.

Why should I care? (OK, I know this is a fingerprinting risk, but please explain that to the user you just popped up a message box at.)

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