I think you hit the nail on the head. They avoid potential windfall profit taxes and they also avoid public scrutiny of their operations. Think about how much scandal Halliburton has been involved in.

Are all their corporate records going to magically disappear from US shores and reappear in Dubai, deep in an unreachable warehouse that also stores lost arks?
Hmmm ...

They just might. What many folks don't realize is that Halliburton has many huge business dealings with the axis of evil. Dubai is a perfect location to facilitate those business activities. Halliburton also deals in portions of the arms business, so what better place to be....

All sorts of irritations dissappear:

Tax
Windfall taxes
Congressional Investigations and oversight.
Meddling Journalists and authors.

All sorts of irritations evaporate when you throw your lot in with a feudal City State. All sorts of new opportunities arise.

Welcome to the 5GW world of Corporate Wars: Once you are free of Congressional oversight, you can buy a private , mercenary army and then do what you want.

Corporations are now living, breathing, intelligent entities. They supercede the National Governments of old.

Like any living creature, they exist to grow and breed.

They look after themselves.

Funny though: They spent the last decades waving old glory.

And now they are leaving...

Funny that... Who would have thought they would have put personal survival above the common wealth of the nation that nurtured them?

What did you really you expect from a VP who 'had other things to do' when grunts went to Vietnam?

'So long suckers and thanks for the cash'.

Seems to me that William Gibson's view of the immediate future was right on. See Neuromancer, Mona Lisa Overdrive, etc..