It seems a number of MLS team supporter groups are enacting, expanding and/or planning to beef up enforcement of rules forbidding people who wear jerseys or shirts of teams other than the home team in the supporters section. According to a discussion on Chant, Third Rail is among them. This is in anticipation of having people show up in Argentina 10 Messi jerseys, because that's not technically the opposing team. I have no issue with people being allowed to wear whatever they want in the rest of the stadium. I might will judge them, but I have no interest in policing it. But I also cannot imagine wearing anything but home team colors in the supporters section.
There does not seem to be a lot of controversy over these rules, I mostly find it interesting this is even necessary.