User blog comment:AYET/An awkward situation - Rayleigh at GR12 after Luffy's departure/@comment-1595565-20121022061112

Like you said, they had the execution and then the cleanup to worry about. I wouldn't be surprised if Rayleigh just slipped Kizaru's mind. Or, even after all that stuff, it would probably be too much of a chore for the Marines. Rayleigh isn't exactly weak in his old age, as evidenced by his strength and swimming hundreds of miles through the calm belt with a cold while beating up giant ocean-dwelling serpents. If even one admiral, regardless of who it is, were sent (because let's face it, ground troops are just haki fodder at this point) to take him in:

1. He wouldn't go without a fight

2. There would be some extensive collateral damage.

Imagine if Akainu were sent to bring him in and the battle that would ensue. In the end, half of the archipelago would be consumed in lava while the other half would be on fire. Given the circumstances, it would actually be counterproductive to capture Rayleigh at this point in time. He isn't harming anyone, so he's not really a problem anymore. He's actually in the best situation he could be in. No one can touch him because of the collateral damage it would cause. But, now that I think about it, by the same token, it might prompt him to go quietly, since he probably wouldn't want other people to get hurt. It's the perfect stalemate. This is an interesting piece, I never really thought about it.