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I'm sorry to hear about your situation. The main problem is that Arizona is an "at will" employment state. At-will employment means that without a contract, you have no contractual or other right to employment with the company. The company is entitled to fire you for any reason: a good reason, a poor reason, or no reason at all--as long as the company does not fire you for an illegal reason (race, gender, age, religion, etc...). But it extends beyond firing, to hiring, promotions, demotions, wage cuts and raises, disciplinary actions, and even scheduling. Unless you can show that this was done in violation of a contract, union agreement, or a clear violation of an unambiguous and binding clause against the employer, or that it was done because of some minority status (age, race, gender, religion, disability) that you have, then they do have this discretion.
Termination based upon a criminal record is legal (although in certain instances, such as yours, I agree that it doesn't make any sense). Even if you had it expunged from your record, that doesn't mean that it didn't happen (and your current job would still have the information that it happened in the first place). The best thing that you can do is talk to the supervisors and managers above you and see if there's some sort of exception that can be made. Explaining the situation, that you're trying to get it expunged and that it certainly was an unusual and abnormal matter (that would never happen again) could help. There's no legal right that you can rely upon here, as the employer is still within their rights to terminate under these circumstances. But appealing to their humanity and not trying to sweep it under the rug, but seeking for them to give you a chance would probably be the best option.
Hope that clears things up a bit. If you have any other questions, please let me know. If not, and you have not yet, please rate my answer AND press the "submit" button, if applicable. Please note that I don't get any credit for my answer unless and until you rate it a 3, 4, 5 (good or better). Thank you, ***** ***** luck to you!