Sometimes the point of following the law

....I am pleased to announce that Kevin McAleenan, the current U.S. Customs and Border Protection Commissioner, will become Acting Secretary for @DHSgov. I have confidence that Kevin will do a great job!

— Donald J. Trump (@realDonaldTrump) April 7, 2019

Matthew Yglesias notes that maybe Trump isn't following the law in appointing a new acting Secretary of DHS:

The key issue is 6 USC §113(g) from the statute governing the Department of Homeland Security, which contains language that overrides the Federal Vacancies Reform Act, the law that typically dictates appointing acting officials. The statute states that if an acting secretary is needed because of a vacancy in the secretary of homeland security position, the undersecretary for management becomes the acting secretary[.]

You can go to the article itself to read the relevant section of 6 USC.  The basic legal argument is that this statute specifically exempts the acting Secretary of DHS from being appointed under the FVRA, making the appointment a statutory succession.  Yglesias walks through all this, then says:  but so what?  Trump's DHS will still be Trump's DHS, in the end:

In the case of McAleenan versus Grady, by contrast, there are no particularly obvious policy stakes. They are both more or less homeland security professionals (his background is in airport stuff, hers in the Coast Guard) who were tapped for political positions by John Kelly and presumably align well enough with Trump philosophically to still be around in important jobs.

Even to the extent that there are policy differences between them, there’s no equivalent to the special kind of Mueller oversight role or big question about which party runs an agency in play.

Maybe the most important "so what" of this situation is the rule of law.  Trump, by all reports, fired Nielsen because she put the rule of law above his demands of her.  Upholding the rule of law in all cases, even the smallest ones, is, or should be, of supreme importance.  Not because the ultimate happens, and you are pursuing the devil while chopping down the forest of laws to do it, as Robert Bolt had Thomas More argue (and More was actually more of a prick than heroic, but anyway....), but simply because setting aside laws on grounds of convenience or because "it makes no difference this time" soon sets a precedent for the law making no difference any time.

Besides, in the context of the stories of Trump's demands to defy court orders, he has clearly reached down to the Border Patrol in hopes of finding someone directly sympathetic to his illegal aims.  Maybe Grady would be as anxious to do the boss's bidding as McAleenan (or maybe neither will defy the courts so openly); the point is, we must uphold the laws in all matters, and we must expect our President, who is the ultimate administrator of those laws, to do the same.

In the end, there's not a whole lot of difference in the argument to say "it doesn't matter" in this case, and to say Trump only meant MS-13 gang members when he called all immigrants to the U.S. "animals."