Yes, it's legal. If you don't have a labor union to protect you, or a written employment contract specifying that there needs to be "good cause" to fire you, you have very little job protection as an "at will" worker, and you can be fired without notice for any reason or not reason, with the supposedly commensurate right to quit on the same basis. Similarly, you can be demoted, have your hours and/or salary reduced, etc.
I've answered similar questions on Avvo hundreds of times over the last 6 months, and it seems to be a widely held misimpression that as long as you're going your job, your employer can't fire you, or reduce your pay, or do something else to change your employment terms for the worse. Nonetheless, that's how it is. You don't have a right to be employed, or to notice, or to a reason for termination if your employment is "at will," as most people's employment is.
If you want that to change (France, for example, has real job protection for their workforce, they're not "a will"), always vote, and always vote for the most progressive/liberal Green pro-employee candidate you can, and tell your elected representatives in the U.S. Congress and in the CA Assembly and Senate that you want job protections for all workers. Republicans consistently make laws favoring the businesses that give them big contributions and not for their individual constituents, Democrats are much more likely to make laws favoring the little guy, and progressive/liberal/Green candidiates make the Democrats look like Republicans.
Until the laws change, support labor unions any chance you get. They make working conditions better for everyone. Try to get to the point where you can demand an employment contract.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.
Unless you have an employment contract, or are covered by a collective bargaining agreement, your employer has the right to reduce your rate of pay.
In fact, as an at will employee, you can be fired at any time for any non-discriminatory reason.
If the economy gets better, and jobs become more prevalent, you may be able to bargain for a better rate, or get a job with a new employer who is willing to pay more.
PLEASE NOTE THAT THIS RESPONSE SHOULD NOT BE CONSTRUED AS A LEGAL OPINION OR ADVISE, AND DOES NOT CREATE AN ATTORNEY-CLIENT RELATIONSHIP. IN ORDER TO RENDER A LEGAL OPINION OR ADVISE, THE RESPONDING ATTORNEY WOULD NEED FAR MORE INFORMATION THAN HAS BEEN PROVIDED, AND WOULD NEED TO BE RETAINED PURSUANT TO A WRITTEN FEE AGREEMENT.