Also, legality isn't the only difference in the VW case. With VW, they had a "good emissions" mode. They enabled the good emissions mode during the test, but disabled it during regular driving. It would have worked during regular driving, but they disabled it during regular driving. With compilers, there's no "good performance" mode that would work during regular usage that they're disabling during regular usage.
It sounds like a mismatch of definition, but I doubt you're ambivalent about a behavior right until the moment it becomes illegal, after which you think it unethical. Law is the codification and enforcement of a social contract, not the creation of it.
There are many cases where I think that. Examples:
* Underage drinking. If it's legal for someone to drink, I think it's in general ethical. If it's illegal, I think it's in general unethical.
* Tax avoidance strategies. If the IRS says a strategy is allowed, I think it's ethical. If the IRS says a strategy is not allowed, I think it's unethical.
* Right on red. If the government says right on red is allowed, I think it's ethical. If the government (e.g. NYC) says right on red is not allowed, I think it's unethical.
The VW case was emissions regulations. I think they have an ethical obligation to obey emissions regulations. In the absence of regulations, it's not an obvious ethical problem to prioritize fuel efficiency instead of emissions (that's I believe what VW was doing).
What if I make sure to have a drink once a week for the summer with my 18 year old before they go to college because I want them to understand what it's like before they go binge with friends? Is that not ethical?
Speeding to the hospital in an emergency? Lying to Nazis to save a Jew?
Law and ethics are more correlated than some are saying here, but the map is not the territory, and it never will be.
My point though, is that in general, when there's not a right that outweighs the law, it's unethical to break the law.