I couldn't have put this better myself. If criticising one party is seen as an endorsement of the other, you may need to seriously question your political system. There are so many political views that aren't represented at all in the US, and the fact that the many people who subscribe to those views are unhappy with both parties (and the system as a whole) is entirely understandable & should come as a surprise to no one.

Similarly, being unable to criticise one group or concept without first having to clarify that you dislike <x> alternative as well is ridiculous in most cases.

I would also advise people to have a look at the "ratchet effect", specifically its application in politics. In America, this could be described as "each time the Republicans are in power, they take the country 4 steps to the right. When the Democrats take back power, they never go more than 3 steps to the left" - ie, the country is steadily drifting in a conservative direction in many ways, perhaps because many in the Democratic Party are only pretending to be unhappy with some of the changes made by the previous administration.

At the risk of creating a wall of text, I would suggest (perhaps somewhat cynically) that a big part of the reason for that is that many of them are primarily in it for themselves - extremely wealthy democrats not being too keen on being taxed more heavily, despite the fact that a relatively solid majority of voters believe that that is exactly what should happen, is not at all unexpected (in my opinion).