You can smoke regular tobacco through a bong, lol.
Would they arrest you for bringing in a cigar humidor aswell?
Just because 99.9% of the people use them for weed doesn't mean you're going to, and even if they think that they need to prove it. A clean, unused, bong wont rat you out to the customs/police. Don't worry.
EDIT:
Here's a prank proving just that; https://www.youtube.com/watch?v=I8NVvU6QaPs