Secondly, America passive under the occupiers' boot is an old trope, and arugably first used outside SF by Sinclair Lewis, who wrote It Can't Happen Here, about a Fascist takeover in the 1930s. This was an actual fear of many folks in that decade - retired Marine General Smedly Butler even testified before Congress about a conspiracy to remove Roosevelt and his New Dealers in favor of oligarchic capitalists. You can look it up. Dick's book isn't even unique in SF; there is Cyril Kornbluth and Fred Pohl's Not This August and more recently, Maureen McHugh's China Mountain Zhang, though in both of those stories the occupiers are Communists. The fear of a Fifth Column goes back a long way, and had a kernel of truth to it; not all the Japanese in the internment camps were innocent of collaboration with the Japanese, and quite a few members of the German-American Bund went to jail after Pearl Harbor for being a little too cozy with the Reich. Even Star Trek went there - have you all forgotten why Kirk's Depression-era girlfriend had to die? I could go even further down the rabbit hole and posit that in an America where FDR was praised for his Mussolini-like actions, and the Germans were the only ones with the atomic bomb, a lot of folks who supported Roosevelt (or Long, or Coughlin) might sign up with the winning side. You may not want to believe these things, but I submit that Dick was a lot closer to that time than we were, and better able to craft them into a novel fans found compelling enough to give the Hugo for Best Novel.
The bottom line is that these folks are making assumptions based on the statements of a whole cast of unreliable narrators, who may not actually know the truth themselves and sure aren't blurting it out in front of God and everybody in a time and place when the truth is liable to get you beaten half to death and then shot. I personally thought the first two episodes were excellent, possibly better than the book (which I haven't read in decades), and chock full of delicious, hallucinatory surrealism.