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Your Rights as an At-Will Employee
Even if you are an at-will employee, you still cannot be fired for reasons that are illegal under state and federal law. In these situations, the government has decided to make an exception to the general rule of at-will employment.
For example, if your employer is subject to federal and state laws prohibiting job discrimination (as all but the smallest employers are), you cannot be fired because of certain characteristics, such as your race, religion, or gender. (For more information on discrimination, see
Your Rights Against Discrimination and Harassment.) Similarly, you cannot be fired because you have complained about illegal activity, about discrimination or harassment, or about health and safety violations in the workplace (see Nolo's article
Assert Your Safety Rights Without Fear of Retaliation). And you cannot be fired for exercising a variety of legal rights, including the right to take family and medical leave, to take leave to serve in the military, or to take time off work to vote or serve on a jury.
At-Will Agreements
To protect their right to fire at will, many employers ask job applicants and new employees to sign a written statement agreeing that they are (or will be) employed at will. Such a statement might appear in an employment application, an employment contract or offer letter that the employer asks you to sign and return, an acknowledgment form for an employee handbook, or elsewhere.
When You Should Sign an At-Will Agreement
Theoretically, you don't have to sign an at-will agreement -- but most courts have held that your employer can fire (or refuse to hire) you for failing to do so. For this reason, most applicants and employees simply grit their teeth and sign on the dotted line.
Even though you may not have much choice about signing an at-will agreement, that doesn't mean your employer will rely on it to fire you without a good reason. Savvy employers know that they have nothing to gain by firing employees arbitrarily. Instead, employers are often motivated to work through issues with you before resorting to such drastic measures.
When You Should Think Twice Before Signing an Agreement
Be wary of signing an at-will agreement if you relied on your employer's promises of continued employment when you decided to accept the job. For example, let's say that your employer promised, during the hiring process, that it would give you at least one year to learn your new job and that you would not be fired during that time. If that promise influenced your decision to take the job, you should not sign an at-will agreement contradicting the promise. Virtually every court will treat a signed at-will agreement as the final word on the subject, no matter what your employer said to you earlier."
http://www.nolo.com/legal-encyclopedia/employment-at-will-definition-30022.html
I'm wondering what law has ever prevented a business from firing someone simply for saying disagreeable things, unless the employment was specified otherwise...