Sure, but scamming the gullible is still both criminal and immoral. And similarly intentional dangerous misrepresentation of facts like this should be criminal (and maybe already is, I do not know American law that well) even if you would have to be an idiot to actually believe it.

It's not really medical advice in the sense someone would normally think of it, it's vacation safety advice. While one should probably treat the words of any salesperson with extreme skepticism, a lot of people don't, and those most at risk (the elderly) use the official word of their cruise company as their main source of safety advice (usually of the form "get this vaccination" or "beware this part of town" or "this kind of tour operator on shore is a scam"). And regardless, salespeople are absolutely expected to be responsible for the claims they make, regardless of one's views on how much they should be trusted.