Depends. Technically, if you live in an "at-will" work state your employer can fire you at any time for any or no reason. Regardless of if you work in an at-will work state most employment contracts include verbiage allowing the employer to let employees go for any reason. Now if someone's firing violates any laws like those against discrimination against women, minorities, religion, employee's age, national origin, disability, or other protected groups federal and state laws will usually supersede your employment contract. Most employers will provide a reason for termination even if it is a lie to avoid legal action.
No - under the wrongful dismissal clause, an employee needs to be given a reason for termination, unless they have signed a contract that allows the employer to do otherwise.
in england its a major "nono" im guessing from the comment above your american so each state can have diferant laws but im pretty sure they need to give you a reason
If you request the reason they have to give it to you