They know that if they begin to drill, they will get sued by some environmental group. It's not about whether they have the legal right (assuming they have any right), it's about how they will be challenged by having that legal right. Why bother? It's more cost effective to do nothing.
Quote from cybtropic:
I just told you they are not using 80% of the leases they hold. They don't need permission to drill on that land / in that water.