people are more likely to respond to disagreeable things that they have been hearing their entire life, like "America is the best country in the world." this is very easily shut down by discussing its history of colonialism, discrimination, and slavery. he ruined is experiment by spouting nationalistic, blindly patriotic, rhetoric.
some dude waving an ISIS flag is clearly just being a contrarian. that person is also less likely to be argued with because overall, people are less familiar with it. it's not that people are supporting ISIS by not confronting him, it's that they are actually more suspicious of him and less prepared to argue with him.