I think when its spelled with the lower case letter it is referring to a concept of that country that encompasses all the 'worst' of it, and ignores the best. I find myself doing this sometimes and the way I thought about is.. well, there is no "American" as a nation, there is only "America" as a continent and then "America" as an abbreviation for the USA, which is a complex entity full of extremes and everything in between. "American" can technically refer to other countries as well. But america or murrka or amerika are used to refer to the concept of the country that the stereotypical dumb inhabitants think they live in, not the actual, concrete territory with all the complexities of the people living there. Of course, the criticism here is that even these "dumb inhabitants" are an imagined stereotype and not a reality. So its a "we think that there are people there who think stupid things" situation. I think this is actually a big problem for anyone criticizing the USA. This language frames the conversation rather poorly and turns many people off, so that a genuine debate and understanding becomes impossible. I know I wrote a bunch of stuff here that isn't really on topic, but I think that on these forums we have a very diverse crowd and I'm hoping this little paragraph might make one side think about the language they are using and the other side understand what the others are trying to say.
Yes, I studied communication and semiotics
Sorry for this interjection