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".