Political anger is rooted in moral sentiment. People justify their anger because they believe bad, harmful, or even evil things are being done. Conservatives and liberals see their anger as righteous indignation. The difference between the two groups are their definitions of right and wrong.
While we hear a great deal about moral relativism, it is in some ways a chimera. Relativism is a philosophy of convenience, allowing people to ignore moral obligations they believe interfere with their pursuit of personal pleasure. The side of the political spectrum most supportive of relativism, liberals, are just as capable of righteous indignation as any member of the religious right. It's not that liberals are relativists and conservatives are moralists, it is that they fundamentally disagree about what is right and what is wrong.
Since at least the 1960's the left has assumed the role of moral leadership for American society. Prior to that time, the moral consensus was shaped by widely understood Protestant values, such as marital fidelity, the work ethic, self-reliance, thrift, and personal decorum and responsibility. To see an example of this consensus watch a Doris Day movie from the 50's or a Leave it to Beaver re-run. With the freedom marches and the victory over racial segregation in the 60's, we saw one of liberalism's finest hours, and its rise to the place of defining acceptable and unacceptable behavior in our society.
Today they define what should be legal or illegal (Same-sex marriage-legal, hate crimes-illegal), what is important (stopping global warming) and what is unimportant (religion). What becomes disturbing is the lack of a real opportunity to debate the merits of these beliefs, and to question the qualifications of those defining these important decisions about our values and our laws.
Sadly, apart from their absolutely correct opposition to racial segregation, the left has defined its moral causes by the distorted values of the enlightenment. Over time, the anger of the left was directed at most of the previously held values of the American middle class. Eventually, they added the issues of environmentalism, sexual orientation, and economic egalitarianism to their list of things to be mad about. So, while the left has achieved moral leadership because it has achieved political leadership, that doesn't mean that its proscriptions for our values and behavior are right or beneficial.
The values of the previous generations and of the social conservatives today were built upon the foundations of the Bible and natural law. These values have been proven in the crucible of human history. They are a significant reason that America has been as successful as it has been. We turn our back on these values to our own peril, and a great deal of the anger generated by the Tea Parties is founded on the concern that we are endangering the future of our society by the rejection of these fundamental values.
In the end, the question isn't which group has the right to be angry. The more important question is which group has the right values upon which we should build our society. The testimony of history supports the Tea Party.