From the perspective of another team's fan. Back in 2009, Dante Wesley of the Panthers put a cheap shot on Clifton Smith and essentially ended Smith's career. If the NFL came out and said the Panthers had a bounty system going, there's no amount of arguing or evidence that could convince me that it wasn't true.
When a Panther fan hears that there was a bounty system in place, they're going to think of the cheap shot Roman Harper put on Steve Smith last year and think, "Oh, that explains that cheap shot." Their mind will be understandably made up. They're not going to need more evidence. Same with Vikings fans who thought the Saints were trying to injure Favre. I'm sure the Falcons fans have some example as well, but they're the Falcons, so who cares.