It's an international thing. TSA will allow them domestically, but Australia/New Zealand won't allow them. (Which I find very amusing since they produce so much of our yarn!) I'm sure there are other countries that won't allow them either.
However, other countries will allow lighters in checked baggage and the US they are not allowed.
My recommendation is to carry bamboo needles and keep a low profile. Be prepared to either mail your needles back to yourself, or just leave them with security. Check on an expat or travel board to see if they are "legal" on the flights.
Oh, and don't leave your knitting on the plane because you'll never find it again!