I had to look up how they handle marijuana laws since that has the _look_ of the DOJ doing just that.
'In each fiscal year since FY2015, Congress has included provisions in appropriations acts that prohibit DOJ from using appropriated funds to prevent certain states, territories, and DC from "implementing their own laws that authorize the use, distribution, possession, or cultivation of medical marijuana"'[1]
So in that case it's Congress that prohibits the DOJ from enforcing a federal law. So your point stands in that the DOJ may not be able to unilaterally decide not to enforce a law, but apparently congress can sort-of extort them into ignoring laws? Oh America.
When he first won office, I figured that it would happen within six months. Given that he was impeached twice, I was almost right, but it didn't happen until Democrats won the House. Even most the "old establishment" Republicans ended up backing Trump. Now there are none of those remaining.
> Although President-elect Donald Trump could choose to not enforce the law...
Which is ridiculous. It's the executive branch's function to "take care that the laws be faithfully executed" [1]. The president's DOJ can't simply refuse to enforce the law. There's some debate over whether this applies to 'enforcement discretion', in that the president doesn't have infinite resources to perfectly execute the law and some things will slip through, or whether the president can decline to enforce a law that he believes to be unconstitutional before the supreme court declares it to be so.
In theory, no, the president can't simply decline to enforce a law, congress would then be able to impeach and remove him. In practice, though it happens a little bit all the time. And even if this was black and white, I don't know that there's anything that the incoming president can do that the incoming congress would impeach him for.
[1] https://constitution.congress.gov/browse/essay/artII-S3-3-5/...
[1] https://en.wikipedia.org/wiki/Article_Two_of_the_United_Stat...