I completely get you, about wearing the opposition's jersey not being a green light for abusive behavior - however I do see it as a slippery slope. If the cops are willing to disguise themselves as ordinary members of the public, then want to get more of an edge, then they wear the home team's jerseys. Then they want to get more of an edge, then they wear the away team's jerseys. They they want to get more of an edge - do they start acting obnoxiously, in the hope that anyone who might be slightly acrimonious towards an obnoxious away fan has the best chance of committing an offense?
The fan code of conduct stated below contains such a wide range of things, that if the cops were feeling so inclined, they could create ways to get people out of the stadium, which fall short of any criminality.
If this is as big of an issue league-wide, then the NFL should be looking into designated away fan areas, like soccer teams do worldwide, where the instance of opposition fans traveling to games is more prevalent. The Washington Huskies - Oregon State Beavers game I went to at Centurylink in Seattle was done very nicely - Oregon State had a huge section of traveling fans, and the back and forth banter between the fans was very appropriate, and added to the atmosphere. However the issue here is that if all seats for a visiting fans section cannot be sold, then you are likely going to have empty seats, which probably doesn't fit the business interests of the ownership.
If I'm going to a home game for my team, and a guy is sitting right behind be shouting encouragement to the other team's players and whooping and hollering whilst my team are on offense, I'm probably not going to enjoy the game as much as I would if they weren't there. I'm not going to start trying to fistfight the guy though, but having that presence there may well diminish my experience, and if that presence is only there because the guy is an undercover cop, I'm going to be even more pissed.