Is it legal to be let go without prior warning, explanation or faults?

If you are an at-will employee, yes.  If you have an agreement stating that you are entitled to discipline, then no.  Check to see if you have any written policy.  However, likely, the employer made clear in it that the policy was not creating any terms of an employee agreement.  If so, unless there is an agreement to the contrary, you are out of luck.

Unless there is an employment agreement (including a union contract) to the contrary, then yes--employers may fire people any time, for any reason or no reason, without warning. The only other signifcant exception is they may not fire people for discriminatory reasons; e.g. no discrimination in firing against a race, religion, gender, disability status, age over 40, etc.

Most employment relationships are what is known as "at will".  This means that you can work for an employer, or not, your choice.  In turn, the employer can hire or fire someone for any reason or no reason whatsoever.

The only exceptions to this are:

1) If there is an employment agreement or contract (which governs termination);

2) If there is a union or collective bargaining agreement (which governs termination);

3) If there is an employee handbook or other official policy statement (which governs termination);

4) If discrimination is a factor (there can be no termination for reasons of race, religion, age, disability, sex, national origin).