they are legal until you smoke weed out of them. if there is any weed residue or anything on them then its illegal
not necessarily. if you travel from canada to pensylvania, or other of the states with strict paraphernalia laws, then they CAN charge you for possession of paraphernalia, confiscate your pipes, seize your cash, etc.
even if it's clean, never used for mj, they CAN charge you, just to seize your property. and getting your property back after it's seized is a pain in the ass... and sometimes impossible.
it depends on the cop, the DA, your record, etc etc etc....
here's a theoretical situation:
they find the piece at the airport when you arrive for some reason. it's clean but they charge you with importing drug paraphernalia, a felony in the particular state you are in. you have 400 dollars cash spending money for your weekend vacation. they seize the money, and will argue that since you were charged with importing drug paraphernalia, the money was legally seized because it could be used in the future to buy drugs.
believe it or not they CAN argue that seizing is legal because it could be used in the FUTURE for a crime. like minority report, or some shit.
it's happened before, there's precedent, i can't remember what happened.
either way, getting the money back will be a pain in the ass, after you spend 36 hours waiting to talk to a DA, who pleads you down to a misdemeanor, if you give up the money, that was NOT gonna be used for drugs. even if you were planning to buy a gram for 20 bucks, they'll take the whole 400.
i don't think there's a pipe in the world you can't find somewhere else. they might all be slightly different, but no piece will ever go on a plane with me, EVER. just buy one when I get to where I'm going if need be, but i won't risk myself at airports, ever seen locked up abroad?? HELL NO!