I knew I was making a mistake when I couldn't think of a concrete example off the top of my head, so pulled that generic one out of my bum instead, based on some vague memory of an example that may even be a phantom memory implanted by some other poop comment. Withdrawn example.
I feel as if there aught to be a name for the thing I was trying to illustrate, where one ignores faults in a beloved while criticising a hated in similar terms. Perhaps I should look on Wikipedia...