Actually, since the beginning, TSA has given FSDs leeway in making such local policies that are more restrictive than what TSA itself requires. You just can't make something less restrictive. So long as the FSD in question is capable of justifying it to Washington's satisfaction, then it typically stands.

I would note that it would be next to impossible to justify any kind of massive difference, such as... oh... Nashville adding red bonnet-style hats to the prohibited items list, or something, and Seattle deciding that only Hefty-brand plastic bags can be used to take LGA through the checkpoint. Typically, these differences are minor, like with the iPad stuff, or whether or not shoes need to be in a bin or not. TSA's official baseline rule is that it doesn't matter if shoes are in a bin or not. Some airports have made local policies that require shoes to be in a bin, others that require them to be directly on the belt. While I consider such an argument to be silly, I'm sure they have what they are just absolutely sure are good reasons for having such local policies.

That's how I understand it, anyway. We only have one local policy in our hub/spoke that I'm aware of that differs from what TSA brass says to do, so things are kept fairly simple on our end.

Incidentally, I've also heard from passengers that Dulles and Las Vegas also require small electronics, like iPads and Kindles and Nooks and such, to be removed from carry-on to be put through the x-ray.

That would be a correct assumption, based on my own observations. If I had to just completely make up a number and pretend it's the least bit statistically relevant, I'd say that it's four clip-on styles to every one button-on style. Though lately the all-plastic clip-on style has become more popular, too.