That's a great question and one that I've thought about too. When there is an anemone in the tank, they'll usually stay in it or close by. They're not out swimming constantly like the other fish. So, theoretically, they MIGHT have a lessened chance of getting these parasites during the parasite's free swimming stage since they're not in the water column as much. Clownfish may also develop a thicker slime coating when living with an anemone than when they are without. So, the thicker slime coating may help too.