Being a fan of these United States while being an anarchist is actually pretty simple.
These United States got closer to the truth than anyone else in history. We proved that even the best government on earth is ultimately going to be usurped by authoritarians.
If the only governments we ever had were the likes of the fascists and the Soviets, it would still be reasonable to think that it's possible to get government right.
Now, we have proof that it's impossible to get government right. Now we know that explicit language like "...shall make no law..." and "...shall not be infringed..." aren't enough to protect liberty from the natural reality of government.
Now, smart people know that government isn't a flawed but fixable idea. Now we know that the only sensible and morally defensable way to continue to exist is to abolish all of the murder, theft, and rape machines that we call "governments."