Disney, like most businesses/places, has signage/disclaimers in place, saying that if you're experiencing any of the following (list of symptoms like fever, coughing, etc.) then you should not enter. That being said, medical privacy issues are extensive, so I doubt Disney would, say, see someone coughing or whatever and say they should quarantine or can't come in. The temperature reading provides a specific, medically validated and measurable figure that they can use to deny entrance, something that doesn't have room for argument or interpretation like a vague symptom like a cough. But even that, if a guest was a complete jerk, they could load up on tylenol to get their fever down before going through the main gates.
Unfortunately, beyond the temp screenings and the disclaimers there isn't really much they can do.