Because even though he might be, there are people who just can't get vaccinated. Immunosuppresed patients for example are the weakest ones already and they can't get any protection by a simple flu shot or anything like that. If one of those gets a bad infection his life is in some serious danger because someone else was believing in some risks that were mostly made up by people claiming to be able to "free you of those risks by detoxing you" or stuff like that.