US Constitution Section 2 Article 3: "[The President]... shall take Care that the Laws be faithfully executed..."
The enforcement of the law is not at the president's discretion. That would make Congress powerless. Congress is not powerless.
Your argument reminds me of the clip where the guy who had his license revoked for DUI is surprised when the car starts anyway. Just because a law says a thing, a person still has to action something. If the boss says don't do it, you don't do it. Don't rightly matter if the law says you should.
Can you cite an example? If you're talking about the Biden admin, here is what the AG said:
"I do not think it the best use of the Department’s limited resources to pursue prosecutions of those who are complying with the laws in states that have legalized and are effectively regulating marijuana..."
I.e. exactly what I said is defensible in certain scenarios – and indeed must be defended, potentially in court. It is not POTUS simply saying "I won't do it."
And I don't think Garland's longer, winding way of saying what he said doesn't reduce to "I won't do it".
The only real check against the president/executive branch is the legislative branch having the power to impeach the president and get them replaced, or by legislatively dismantling or changing the internal rules of a department. They lose their jobs, but aren't liable for anything or breaking any laws I know of. Just the same as states can boot out judges or prosecutors if they don't like how they operate, and police departments can fire officers if they don't like how they operate. At no point did cops or prosecutors or judges break the law by not enforcing the law, they merely piss off the state that is funding them and now losing additional money due to lack of enforcement and prosecution, and I don't know how the president or executive branch is any different.