While the Packers fan who had his sign ripped down before the game by Cam Newton apparently threatened to take legal action to get his money back, it seems that he may not have much of an argument.
According to the banner and flag policy on Panthers.com, it clearly states the following:
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"No banners or flags expressing support of visiting teams may be affixed to or displayed on any surface. Hand held signs or posters that are in compliance with the balance of this policy are acceptable."
Well, there you have it. The Carolina Panthers were apparently still generous enough to replace the fan's banner, but they didn't have to.
(h/t Pro Football Talk)