Original post quoted an e-mail from Port of Seattle
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Clear plastic cups are not allowed through security. Drinks in solid plastic, metal or glass are allowed if they are in spill-proof containers that can be placed on the x-ray conveyer"
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that implied clear plastic cups were not allowed either in the walk-through or in carry-on. I can easily see a TSA-type misinterpreting the above words to disallow clear plastic containers in carry-on, even though that's probably not what the text was intended to mean.
The TSA rules quoted from the website are much more clear, and in this case, make decent sense, but we all know how consistent TSA is when it comes to using the excuse of "local policy."
I doubt an x-ray can detect whether a cup is made of plastic or plastique anyway if there's no detonation wires attached, so it's kind of silly. What are they going to do, individually ETD all plastic in carry-ons?