These are oil companies we're talking about, and their business involves extracting a nonrenewable resource — part of our shared patrimony, thus — out of the ground and selling it. They have also acquired for themselves all manner of special incentives and tax breaks (politically motivated disbursements, you might call them) from Congress over the years. So this isn't exactly the same as Congress swooping in from a clear blue sky to steal your lunch money because the politicians think you have too much of it. Unless you think it's a case of, They came first for the oil companies, And I didn't speak up because I wasn't an oil company....I've been surfing around a bit, mostly out of curiosity, to try to find the best defenses of a windfall profits tax, and I haven't had much luck. The most common arguments, I think, are variants of Fox's, namely that oil companies have profited from boatloads of special tax breaks in the past, so it's hardly unfair to turn the tables now that their profits alone are bigger than the GDPs of some medium-sized countries. Additionally, some economists make the case that if a windfall tax is a one-off deal it's not too distortionary — which is a good thing, if not quite a full-throated defense. However, the current proposal isn't a one-off tax, so that argument doesn't pack much of a punch.
If anything, the best defense of a windfall profits tax might be one that no politician can afford to make: namely that much of the tax would be passed along to consumers, which makes it sort of a back-door gas tax increase. That's not a bad thing, though a straightforward increase in the gas tax would be a better way of durably raising the gas tax. But that's still political suicide, so a windfall profits tax is, perhaps, the next best thing.
I'm not sure I find any of this terribly convincing, but there you have it. If you've got a better argument, leave it in comments.