It makes sense to me that many people don’t believe Confederate ideas and iconography are treasonous or even disloyal because about 100 years ago, i.e., about 50 years after the war between the states, there was a big national reconciliation between the southern states and the northern states.
A lot of people recognized that the generation that fought that war was dying off, so people wanted to honor them on the way out. There were parades all over the country featuring veterans of both sides, and the sense that everyone (or at least every white one) is really on the same side, America’s side, after all. That idea was cemented by everyone fighting together against European enemies in the First World War. The South was rehabilitated as a vital part of America with a beautiful antebellum past to cherish and celebrate in cultural touchstones such as books, music, and movies. And, of course, to mobilize against a common enemy of their own making.
All that peculiar thinking has been passed down through the generations and, as we can see, still persists today in a form much stronger then one might presume, considering it’s been over 100 years since the rehabilitation got under way.
I believe this is a big reason many people see no problem with conflating the confederate flag with American ideals. In a lot of ways, the Confederate States was the United States, albeit with a stronger slavery institution.