Well, besides the fact that you can't fire an *elected* official, as opposed to an appointed police chief - which is why elected sheriffs are so important to our Republic - are the following facts:
1) They take an oath to follow the constitution as the highest law in the land, any State laws to the contrary not withstanding. The 2nd amendment is part of the constitution, and these laws are invalid. In fact other laws would make following them a felony - civil rights violations and color of law.
2) You can try to claim a sheriff is fired, but the sheriff can arrest your ass. And the posse that is with him will make sure it happens.
Man, I wish our sheriff was a little younger.
And for the support of this Declaration, with a firm reliance on the protection of Divine Providence, we mutually pledge to each other our lives, our fortunes and our sacred honor.