There is an obligation on the part of the executive to enforce the laws passed by congress. The constitution requires that he "take care that the laws be faithfully executed." Clearly there's some room for prioritization and allocation of scarce resources but he can't just ignore them. The Executive's check on the legislative branch is the veto power.
"The Executive's check on the legislative branch is the veto power."
The President also has the "power to grant reprieves and pardons for offenses against the United States". Choosing not to enforce something is in fact a weaker exercise of power than pardon, as it preserves the ability of future administrations to prosecute current infractions.
