Human rights most definitely exist in the United States. Although it has become fashionable to decry alleged abuses of power by the government, the fact is that the United States is still the home of freedom in the world. The fact that these people are free to make such arguments is proof of their fallacy.
Those who don't believe that the US is free should look at other countries. In the United States we are free to worship as we choose, but in the majority of Muslim nations it is illegal to convert from Islam to another religion. Our press is free to criticize our government, even if they reveal classified programs in wartime, while Russian journalists who criticize Vladimir Putin sometimes end up dead and Venezuelan journalists can be jailed for insulting President Hugo Chavez. Our opposition party frets about the government's ability to wiretap conversations with terrorists in other countries. In many nations, the government is able to wiretap at will with no oversight whatsoever.
Our government, even in wartime, gives due process to criminals and terrorists who are US citizens or are captured in the US. The opposition wants to extend these rights to fighters captured under arms in foreign countries. The Guantanamo detainees that are often cited by the left are more akin to prisoners of war than criminals. Did we extend habeas corpus and access to our court system to German and Japanese prisoners of war in WWII? Certainly not! And a key difference between the detainees and POWs is that the detainees are not part of an organized army with uniforms, but are civilian irregulars. As such, they are entitled to very few rights under the Geneva Convention and none whatsoever under the US Constitution.
The fact is that US history is rife with examples of the limitations on individual rights during times of war. Abraham Lincoln suspended the right of habeas corpus, spent money without Congressional authorization, and imprisoned thousands of Confederate sympathizers including members of the Maryland legislature. Lincoln's government even prosecuted members of the anti-war Copperhead faction of the Democrats for disloyalty.
Woodrow Wilson, remembered as the author of the League of Nations, passed some of the most repressive free speech laws in American history. The Espionage Act of 1917 basically made it illegal to criticize the US government or to voice opposition to US participation in WWI. The American Protective League became a volunteer secret police for the Justice Department that informed on dissenters. The Committee on Public Information fed the public propaganda and encouraged Americans to report disloyal sentiments, especially among German and Irish immigrants.
Franklin Roosevelt also had his share of civil rights restrictions during WWII. The internment of Japanese-Americans is common knowledge. Censorship was also common during the war years, along with heavy restrictions on freedom of the press. Before the war even started, FDR had rammed legislation through Congress to pack the Supreme Court with justices friendly to his New Deal programs, short-circuiting the Constitutional checks and balances on executive power.
In contrast, the current government of the US, in spite of an ongoing war and the threat of attacks on our homeland, has not suspended habeas corpus, arrested opposition party leaders, silenced dissent, or failed to abide by court decisions. Instead, any limitations on rights have been remarkably restrained and noninvasive. Those who believe that the US is becoming a dictatorship with heavy restrictions on the Bill of Rights should go back to their history books.