I opt out, too.

>"So wait, 3oz of fluid in a 3oz container is okay.. but 3oz in a 5oz container is not? Is it more dangerous for some reason?"

As much as I dislike the current set of rules, I feel that if you going to have rules, then you need to make rules which are enforceable. "Containers 3 oz or less" is an enforceable rule[1]. Is it dumb? Yes, because a 3.1 oz container is no more dangerous (certainly not more than around 3% more dangerous) than a 3.0 oz container. "Containers with less than 3 oz of liquid" is not not an enforceable rule. How can the TSA person quickly assess the volume of liquid in the container? He or she can't, which is the problem. You could, of course, give the TSA agent the discretionary authority to allow larger containers with "about 3 oz of liquid," but that's just begging for an equal-protection lawsuit.

[1] Actually, it's not, because, as far as I can tell, the TSA determines the volume of a container by the declared volume on the label. If you really want to bring 5 oz of shampoo, you could probably just make a funny-shaped bottle with a label that says "Shampoo, 2.9 fl oz". Though it would be pretty easy to measure the volume of the entire container via water displacement ("Mr. President, we cannot allow... a thick-walled-container gap!")

And one 6oz container isn't ok, but two 3oz containers is?

My favorite part is that empty bottles are always allowed, regardless of size. So you can have a six-ounce bottle with two ounces of liquid in it, and an empty three-ounce bottle, and that violates the rules, but if you pour the contents if the first bottle into the second, it's suddenly allowed, even though you have all of the same materials as before: bottles, liquids, etc. Totally irrational.

Equally amusing - so long as a liquid is frozen, you can bring as much as you like in any size container, because now it's a solid.

I one brought through security the remains of a soft-drink with ice cubes, that I'd transferred into a bottle. They made me drain the liquid that had melted, but the ice-cubes themselves weren't a problem.

That's why it's called security theatre, not security.

I think they are trying to make it harder for you to mix things.

Hogwash. I've taken quart-size lexan containers through security consistently, and then fill them with water on the secure side.

I think they are trying to avoid arguments over whether there's 2.9oz or 3.1oz of liquid in that 5oz bottle.

It's not that it's dangerous, or not, but "something must be done" when someone, somewhere, at some point, mis-used a liquid. It's not really meant to make sense.