Second, while many in the public are furious that Arpaio won't be going to jail, my sense is that their presumption is that he'd have been put away for a long time, maybe a decade for his civil rights violations. The reality is that isn't the case at all. He was found guilty only of violating a judge's order, and at most he would have gone to jail for just 60 days, and possibly less, maybe even much less, for all we know. This wasn't a case of him being sent to prison and throwing away the key. Given that he's 85-year-old, the sentencing judge may have made it as short as a couple weeks. That doesn't assuage the public anger, but I think alters the perception of what was actually at stake.
Third, by accepting the pardon, it's an admission of guilt by Arpaio, who has always insisted on his innocence, and indeed has said his "innocence" is the reason he welcomed a pardon. Well, he no longer can claim innocence. (He will, of course, but it's not true.) This admission of guilt could also have negative impact on Arpaio in any civil suits against him -- though it's important to know that his conviction was for violating a court order, not any actions against the rights of individuals. However -- if Trump's pardon covers all acts by Arpaio (the final paperwork isn't public yet), then that would mean the former sheriff is admitting guilt to all actions. And if Trump's pardon doesn't cover all acts, then that means Arpaio remains at risk for them.
And fourth, and perhaps most importantly, history has shown the public gets outraged at what they consider inappropriate pardons. Most notably, it is considered to be what cost Gerald Ford the presidency after he pardoned Richard Nixon. (The plummet of Ford's public approval was the biggest drop in Gallup Poll history.) No, certainly, "Forever Trump" voters will be thrilled by the pardon, but it's the independent voters and even his more moderate supporters who will likely be outraged. (As I noted, support for the pardon in Arizona was only around a paltry 20%. I suspect the reality of the actual pardon will make Trump's action more visceral.) In the end, it doesn't matter of the diehard support of Trump stays diehard. That's only 20-25% of the public. A politician is an empty vessel with 20-25% support. It's the stripping away of the wavering support that is so problematic for Trump over a paltry matter of keeping Joe Arpaio out of jail for at most 60 days. While marking it an admission of guilt. (Further, though related to this, it remains to be seen if Trump callously making the pardon during a Category 4 hurricane will come back to hurt him. I suspect that to at least some degree it will.)
None of this is to say it's "good news!" that Joe Arpaio was pardoned as a slap in the face to the system of justice, but it's to add a wider view on something that's over and done, and has a more-negative side for both Trump and even Arpaio than is apparent at first glance.
Both of whom should be reminded -- Be careful what you wish for.
Actually, there's a final point, and I think it's the most-important of all.
There have been many legal analysts who have said the big problem with the pardon is that it's Trump sending a signal to potential witness against his administration that they'll be pardoned, so they're safe. But I think that view is surprisingly missing a much bigger point. (And one I've heard other legal experts reference.) By accepting a pardon, a person loses their 5th Amendment protection (since, being pardoned, you can't incriminate yourself). There'fore, someone who is pardoned MUST testify if subpoenaed...and -- most importantly -- do so openly and honestly. If not, if prosecutors believe or can prove they aren't testifying fully, they can be charged and convicted of perjury or obstruction of justice.
By potentially giving potential witnesses the comfort of knowing they might be pardoned, Trump actually caused far-bigger problems for *himself*. And ultimately, that's the main goal here. Yes, I want to see others go to jail for crimes...but FAR above all, I want to see Trump out of office. And , I believe he just helped push that forward.
Consider this, as well. I think repeated "I don't recall" from just a single pardoned-witness, let alone ALL pardoned-witness will have a difficult time standing up against national outpouring of outrage, an aggressive Special Counsel, and the cleansing disinfectant of sunshine. I don't know that for certain, of course. I also don't know if Trump will even be around to give pardons, nor if Mike Pence would risk his re-election by granting pardons like Halloween candy. Nor do we know if Robert Mueller will even need their testimony, but get it instead from lower-level people never charged with anything so they didn't even need apply for a pardon. AND I think the risk of being charged with state crimes is a strong reality here, as well, for everyone under investigation, and I suspect that Mr. Mueller is already working closely with New York state Attorney General Eric Schneiderman.
The point being, in the end, fFor something that was so minor, making a point to his base that was never leaving him anyway, all to save Joe Arpaio from going to jail for no more than 60 and maybe just a couple of weeks, I think Trump dug himself a major hole.