What’s legal and what’s ethical are often two very different things.
We’re talking about the people who come back after they started developing symptoms, or who report that they began experiencing minor symptoms while in the air. That these people didn’t know or suspect that they were sick is not credible. The “I got a cough while on the airplane” stories are not believable.
By getting on a plane knowing or suspecting they are sick but not disclosing that to the airline and other passengers they are putting other people at risk. Taiwan is one of the very few countries with no local transmission of the virus and the more people coming back who are unnecessarily exposed, the greater the risk that the virus could eventually slip through.
Here’s an idea: if you choose to be overseas during a pandemic, you should be prepared to receive medical care where you are. And if you want to come back to your home country, you should do so ASAP, not when you think you might be sick.