Well, the "conch shells" you'd find for sale on Maui are not local - either they are large helmet shells (Cassis cornuta) imported from the Philippines, or queen conchs imported from the Caribbean. Kind of puts a damper on the whole thing, doesn't it?
Helmet shells (the closest thing we have to a "conch") aren't uncommon in flat, sandy areas, where they roam around looking for urchins to eat. As it stands, it's not illegal to take them, nor are they particularly rare. But, if everybody who came to Hawaii went out looking for a "conch" and picked one up, they'd surely become scarce in short order. Also, if you take a live one it's very difficult to get the meat out without either wrecking the shell or stinking up the entire neighborhood. Now and then you'll find a nice dead one, but to do that you'd have to spend hours roaming around in sand with little else to see. As such, I don't think taking one would be advisable.