> it's considered taboo to express a love of the USA
American here. First-generation immigrant. Came from Germany at age 5.
This misses a crucial part of the problem. It is considered taboo to express a love of the USA in certain social circles. In others, it is considered taboo not to express a love of the USA. The problem is that the two sides have very different ideas of what "loving the USA" means. Among the first group (liberals) the USA is envisioned as an inclusive melting pot where all are welcome. Among the second group, the USA is envisioned as a set of values to which one is required to subscribe in order to be included; to include those who do not subscribe to these values would change the character of the nation to the point where it would not longer be the USA. These values include innocuous things like baseball and hot dogs, to abstract ideals like "freedom", less abstract ideals like capitalism, and quasi-religious ideals like "family values". Lately these have started to morph into religious ideals up to and including the (false) idea that an essential part of the national character is to be a Christian theocracy.
So it's not that expressing a love of the USA is taboo, it's that conservatives have managed to co-opt loving the USA and make it part of their brand. Expressing love for the nation, flying the flag, singing the national anthem, etc. are nowadays seen as expressing tacit support for conservatism in general, and the Republican party and Donald Trump in particular. This is the reason that liberals avoid them.
For me personally, I have always felt that some of the common rituals associated with "loving the USA" were kind of weird. Take the Pledge of Allegiance, for example. I get pledging allegiance to the nation, but to the flag? That has always struck me as bizarre. The flag is just a symbol, a token. Why would anyone pledge allegiance to a flag? But to question this, especially as a minor in a public school, turns out to be unwise.