It’s certainly not unethical to take cuttlebones from the beach, and i think it’s probably a good alternative to buying them from the store if you’re worried about cuttlefish being harmed in the making of cuttlebones for birds. That being said, I honestly have no idea if they might carry diseases for your bird. The cuttlebones in stores are probably produced as a by-product of the seafood industry, so for the health of your bird it might be safer to buy cuttlebones from pet shops, but I can’t say for sure. I bet there are vets or bird rehab people on tumblr who would have better answers for you on the bird-health end of things 🙂
Just drop the bones in boiling water and that’ll kill off any potential diseases. I doubt there would be any bird-harming germs anyway, unless a sick seagull picked it up, but boiling them would make sure.
And it’s certainly ethical to take small items you find on the beach that aren’t currently being used by a live animal.