I think for local (car) travel, it's very hard to enforce. I think that's a legitimate concern. It's easy to say to control it, but how?
Air travel, Trains, Busses.. how do we get people on them and safely? How do we screen? Who does the screening? How are they trained?
NPR had an interview with SF's mayor (I think mayor), and she was talking about the Homeless population and the biggest issue they have is that they don't usually follow the rules. They've had a real hard time getting them to comply with guidelines and rules. Even folks who were tested, and told to stay put for a day or two until the results came back, left.
Granted, that's a population that has a high per capita mental health problems, but still. if you can't guarantee compliance, how do you enforce it or guide it?
Many folks talk a big game about doing the right thing, until they are told "no". The whole internet joke about being a "Karen", derives from people not getting the services they demand. i.e. told 'no'.