A pardon is not a magical legal eraser that says you can never be held responsible for anything you do or say again, ever.
Here is the most logical way to view his pardon of Sheriff Arpaio: it is the latest and gravest step he has taken in his continuing efforts to undermine the rule of law. Obviously Trump delighted in fueling the racism of Arpaio’s supporters by pardoning this convicted criminal –he made that clear earlier this week during his repellent speech in Phoenix. But I am certain that is not the main reason for this heinous act.
Trump pardoned Arpaio because:
a) Arpaio was a loyal supporter who hasn't given Trump a reason to turn his back on him (remember how Trump wanted to re-hire Michael Flynn?)
b) Trump sincerely believed he could do it, but then, he thinks he can pardon himself.
c) Trump was told by advisors not to do it, because of politics; the surest way to make him do it.
d) Trump has no grand scheme, other than firing Robert Mueller and pardoning everybody, including himself, when the time comes.
For many weeks, Washington has been swirling with rumors that Mueller already has secured the cooperation of Michael Flynn and Paul Manafort in his investigation of the president. And Trump undoubtedly is more vulnerable to the testimony of these two men than he is to that of any other players in this fearful drama. Therefore, Trump must feel compelled to send this message through Arpaio’s pardon: the president is eager and willing to do the same thing for anyone who might be pressured into testifying against him.
The problem with this analysis is simple: pressuring Flynn and Manafort to testify against Trump with a prosecution deal is the only way to get them to waive their 5th Amendment rights. If Trump waives those rights by pardoning Flynn and Manafort, then they can't refuse to testify under subpoena (literally "under penalty"), because they can't incriminate themselves on those crimes. It doesn't make the case against Trump go away; it doesn't change a thing. No pardon will remove the risk of a perjury charge, and if Trump tried that, it would make the pardon of Arpaio even more clearly a blow against the rule of law.
And here is another reason why this pardon cannot be allowed to stand:
Contempt is an essential enforcement tool that the judiciary can use to keep investigations from stalling out and to help ensure the progress of justice. Trump is effectively nullifying that tool. By pardoning those who defy court orders, Trump would be able to continually frustrate the investigation and prevent Mueller from uncovering the truth.
The only pardon that impedes the investigation, that nullifies it, is for Trump to pardon himself. Setting that aside, the logical conclusion is fairly clear:
Trump and Arpaio share many dangerous beliefs, and given power, both men use it to shamelessly silence and impede their opponents. Trump has realized that he can use his pardon power to bypass the lawyers and judges and investigators he so despises. Arpaio was a test run. Now he will know it works.
But the courts are not required to recognize the absolute power of the Presidential pardon. They have an obligation to the Constitution to see that all parts of the government under that constitution operate as we the people expect them to. Courts are actually quite restricted in how they can wield their power; and I'm of the opinion, more and more, that the courts have gone overboard in deciding what does, and what does not, merit constitutional review; it's a pandora's box paradox that gave us Brown v. Board (now more honored in the breach than in the keeping) and Heller (which sets up conflicts with provisions of the Bill of Rights). The courts, however, show due deference to Congress and the Executive; and they must demand some due deference from those branches, and not just in the matter of deciding what the Constitution allows, and what it does not allow. The courts must have their authority over their orders and rulings, and must not yield it to the executive branch, or they lose all meaning and authority. If the courts take the stance of that Slate article, or the many who think this pardon is wrong but inviolable, they give up their power to the Executive. What Trump has done cannot be allowed to work. The courts cannot waive their authority, and get it back later.
And if Trump tries to pardon himself, he guarantees a Constitutional crisis which will end in his removal from office. But how could the courts stand against that, if they don't stand against this? Because ultimately a pardon is directed at the authority of the court; it is a creature of equity, going back to the common law when equity came from the ecclesiastical courts, and could offer mercy the common law could not. But even equity cannot overturn the rule of law, and become superior to the law. The President is granted broad power to pardon, but if that power is deemed absolute, the President becomes superior to the law. And if the courts acquiesce to that power over their authority, they have only themselves to blame for the outcome; and we can only looks to the courts to deny this Presidential claim.