To come back to what Fallen angel and Amy were saying, I think it's not about being judgmental towards people who have nail fungus, but I think it's true that many people are not aware of this kind of conditions since it's not something you can die from basically (or strongly affects your health), so in general people simply don't care (especially if the person having a nail fungus has been living with this for years, it became a normality for them).
So if people who have this don't want to treat it/don't have the financial resources maybe to do so, well fine, it's their own business, however they shouldn't be surprised neither offended if a service provider asks them to keep their socks on during a booking because at this point it affects not only the client but can also affect the service provider herself. So if the SP doesn't feel comfortable with that and feels scared for her own health, then I think it seems sensible to take measure to avoid cross contamination.
Same for genital warts as example, many people are not aware of them and keep that on like it's normal, but then clients shouldn't be surprised if a SP who provides OWO will only agree to do OW or refuse to provide the service completely.