Just so you know, that's not a position most moral philosophers take, as long as the law is decided by sufficiently democratic procedures.
The fact that you belong to a group of people, and the people decide on rules, means that violating those rules is a moral violation against that community of people.
To say that has no moral weight at all is a pretty extreme position. Now obviously if there's a conflict with another principle, there are times that other moral principle should win. But to say that there is no moral value whatsoever in following the law is not something I think many people will agree with. And thank goodness.
Not that it's going to change my view, but I'm curious if that's really the position they take.
And "sufficiently democratic" basically means freedom of political speech, adult citizens can vote, representatives are chosen by majority rule, elections are fairly conducted and not rigged, and laws are passed by majority rule.
Obviously you can always quibble over details such as unicameral vs bicameral legislatures, single-member district vs. multi-member district representation, gerrymandering, judicial review, and so forth.
But if people are allowed to freely debate and the franchise is universal and elections are free and fair and elections and decisions are based on majority rule, then those are the basic conditions. So the US and France and the UK are sufficiently democratic; Russia and Iran and China are not (despite holding elections).
Edit: actually I do have one other thing to say. It's quite a coincidence that the conditions you laid out happen to be those that were met in the US right about the time that book came out. Feels like it might have been working backwards from the conclusion.
I'm not even defending it. I'm just describing the mainstream ethical foundation used today.