In my opinion, this is most likely not accurate. It's more likely to be a restriction by an employer for them to avoid law suits. There are lots of nurses out there that have CF, and a couple of Docs have been mentioned too. Whether they work with CF patients or not, as a health care provider they are susceptible to air borne illness more than a 'healthy' coworker. Cross contamination comes into play when you attempt to deal with the CF clinics. With an estimated 30,000 + cases, the likelyhood of you working on a CF patient should be low (unless you work in a hospital that hosts a clinic.) Discuss it with potential employers and suggest that if the patient does have CF, that they can work out another RT working with that particular patient. I'd wager it's more about the realization that you could get infected than it actually is about cross contamination.
In any case, tell them to put it in writing. Unless they can demonstrate that it is a policy that they have, they can't just tell you this! This is my humble opinion, only, take it for what value it may have.