More on Selective Enforcement as Legislation

My previous post raised the question in the context of Obama's apparent intent to unilaterally modify his healthcare legislation. But it is an interesting problem more generally. The theory of our system is that the legislature makes laws and the executive enforces them. But laws cannot, in practice, be perfectly enforced, so the executive is necessarily making the decision about what resources to allocate to enforcing what laws. Where can or should one draw the line between that decision and using selective enforcement to rewrite the law?

This is at least the third time that Obama has offered to do it. The first was when, during his first campaign, he said that under his administration federal marijuana law would not be enforced against people using medical marijuana in conformance with state law—a promise that he promptly broke. The second was when he announced that certain categories of illegal immigrants would not be prosecuted. Revising Obamacare is the third.

Imagine the following scenario at the state level. The governor of California proposes a bill to tax cars that burn gasoline but not electric cars. The bill fails to pass. He responds by announcing that he has instructed the state police that they should enforce speed limits strictly against gasoline powered cars but only stop electric cars if they are going at least twenty miles an hour over the speed limit. 

I am not a constitutional scholar and do not know whether there are legal limits to executive power that would prevent such a tactic. It is legal to selectively tax gasoline powered cars. It is legal for the police to devote their limited resources to catching some speeders but not others, for instance by patrolling highways where they believe speeding is a particularly serious problem. Is it legal to accomplish the substance of the former under the form of something like the latter? 

More generally, what are the limits of such an approach? The executive is not entitled to enforce a law the legislature has not passed. But is it entitled to selectively enforce one that the legislature has passed in order to achieve the effect of one that it has not passed?

Comments from those who know more about constitutional law, state and federal, than I do are welcome.