First of all, it is likely Plutarch is being typically unfair here, as i imagine right after this Epicurus may have mentioned examples like helping a friend, or acquiring food in need. But Plutarch uses this to suggest unfairly Epicurus would be okay with things like murder or sexual assault. Notice how Plutarch conveniently doesn't mention what the "anything the law forbids" actually is.
I agree that this is an important consideration. It would have been obvious to Plutarch and Epicurus that the law sometimes forbids in ways that are unjust, and Epicurus' views of justice clearly show that circumstances can change and agreements can and should be broken when they are no longer just to those concerned. I would suspect that those circumstances (when a law becomes unjust) were of as much concern then as they are today, and that prompted Epicurus' examination of the issue in detail in the Principal Doctrines.
So the fact that indeed punishment may come from breaking a law is true in many circucumstances, and one would hope that laws are generally beneficial. But there are many cases when breaking the law is not only the just thing to do, but also in fact brings much more pleasure and beneficial results from and to the community than complying.
So while "murder and sexual assault" are going to always be wrong, there are many ways to break unjust laws that are not wrong in themselves just because a law is involved, and you have to account for both situations in one's analysis.