We in the wider world very rarely hear about American citizens going against the grain, and that fuels the stereotype of one monolithic entity.
Well, I can see that you didn't read my posts for...well, the
entire Bush Administration.

Jeez...I was
full time 'against the grain' for 8 long YEARS. And not being a particular fan of Obama, I'm still kind of going against the grain (although decidedly less so). During the Dubya years, I went against the grain to the point of exhaustion, in fact. At one point, my own President was calling me (and those like me) an un-American terrorist sympathizer right from the podium, simply because I was opposed to going into Afghanistan (let alone Iraq). And I
still refused to back down. The Wars, the Patriot Act, Gitmo, Katrina...GAH! There was
always something to be pissed off about during those 8 years.
And in my actual encounters with Americans, this has been my general observation - there is, politically at least, as much disagreement, if not more, than in the UK. But the reason I know that is that I've travelled extensively to the US, have American family and friends, and interact regularly in a political sense with Americans on here. The majority of my peers do not; the impression America gives the world is "America! Fuck Yeah!" The OP asked why 'people bash America' - that, primarily, is why. A stereotype built on grains of truth but expanded to create a caricature of American nationalism.
And I must say, even interacting with Americans, I can't quite shake the 'arrogant' stereotype

Constantly being told we're doing things wrong because it's not the
American way does grind you down after a while. I have lost track of the number of things about Britain that Americans have described as crazy, dangerous, undemocratic, violating, and downright evil just because we have a different way of doing something to the US.
911 was absolutely the most traumatic thing that has ever happened to us, as a nation, and a LOT of people didn't know what to do with the shock, fear, and anger we all felt.
I think a lot of people outside your borders have trouble with that attitude, too. And not because we don't feel sympathy, or because we play down what happened. It was, arguably, the first major attack on the US mainland since there was a unified US mainland to attack. Most of us here, on the other hand, live in cities dotted with the crude concrete monstrosities that mark buildings destroyed by war. Our parents were born into a struggling country torn apart by the German war machine, and lived through a protracted and intense campaign of terrorism over Northern Ireland.
When you walk around Washington DC, you see lots of military personnel proudly wearing their uniforms. I ate at a diner once in god-knows-where, Nevada, next to a USAF Captain in full regalia. That is unknown in the UK, because in the 80s and 90s it was considered too dangerous to do so with the IRA imagined to be hiding behind every wall looking for targets. War, terrorism, and the loss of civilian life on home soil has been part of Europe's history for so long we were almost desensitised to it. America, on the other hand, was not used to being stuck at home, so your reaction was quite different to ours. We saw a tragedy, an enormous loss of life, and felt great sadness and a desire to help our allies in their time of need. You saw something altogether more violating, more personal; a whole new and scary world in which your homeland was no longer safe.
This discrepancy in attitude is partly why the American response to 9/11 was so hard for many Europeans to swallow. In fact, when Bush was talking about his War Against Terrorism in early 2002, a lot of Britons couldn't help wondering where that attitude had been when the IRA were blowing up shopping centres. We couldn't shake the impression that we were only going to 'war' against terrorism because now it was for a cause the Americans didn't secretly support.
Now, at least we've gotten back to a national debate. It is true (and again, unfortunate) that our nation's biggest whack-jobs are the ones running it...but just the fact that we are truly debating
at all is better than where we were 7-8 years ago, when near lock-step agreement with the Administration was required, simply to avoid being labeled a terrorist sympathizer.
And I must say, the impression of the US is better now in the UK than it has been since Clinton. Obama was and is a very popular figure over here. The vitriol against the NHS during your healthcare debate hurt you a little - most Britons couldn't see a 'debate' to be had on that subject - but in general, your stock's up in the world. There is a general feeling that the US is willing to come to the table as an equal, rather than as Team Fuck Yeah.
As in my previous posts, I am merely discussing general attitudes here, not arguing that you or indeed any particular Americans hold them. I think we are discussing stereotypes here, and so I am deliberately talking in generalities.