Greg responds...

The short answer is that Macbeth & Demona asked for it. The Sisters weren't enforcing their will on the mortals, (well, they were, but Mac & D didn't know it), they were simply granting a request.

Actually, they were aiding Demona & Mac's desire to trade youth for power.
They then slipped the link in by using a specific magic spell for the youth transfer that included the immortality link. Sneaky, huh?

What you need to understand is that Oberon isn't constantly watching. His magical dictate hangs in the air around them. If they can create a justification in their own minds, no matter how superficial, they can magically get away with a hell of a lot. There's a risk involved. What if Oberon found out later? Would he care or regard it as insignificant? Would he buy their justification? Depends on his mood, probably. He's not exactly Mr. Consistent-Letter-Of-My-Law. As it is, I don't think he has found out, at least not so far.

(By the way, the reason that Demona is perpetually 35 years old is because Macbeth was 35 when they made that trade. He couldn't give her youth that he himself no longer possessed.)