Come on, you are a smart enough guy. Scroll up and read this thread. It isn't just those telling us to feel guilty, a lot of people want to play this down and brush it under the carpet (see above like I said, and look outside the window).
There is a guilt driven towards citizens of western states for having pride in their own nations, and it is absolutely everywhere. Some of it comes from people that have moved here, or second generation immigrants. In other cases it comes from native citizens who think we should feel ashamed of our past, or feel too soft to call out those who say we should be ashamed as they don't want to be seen as right wing or racist. It's a really strange phenomenon and has been accelerating this weird culture war. Hence an increasing defensiveness over things like this shirt. Everyone should be able to feel proud of where they come from, but that isn't the reality, and there is a massive double standard over it depending on what country you are from.
You don't even have to leave this forum to see it, but a 2 second google search opens up a whole world of people trying to tell you that you shouldn't be proud of your country and heritage. Look at this clown: