This is probably what you're 'missing'. Asymptomatic individuals generally have much lower viral shedding, and therefore are much less likely to infect others, than those with symptoms in every viral disease I can think of. There's no reason to think COVID is any different. Asymptomatic people certainly can spread, but at least a few case studies have shown that this is less likely than initially reported. Thus by removing people with fevers you are reducing the amount of potential spread. You are certainly not eliminating possible spread, but neither does almost all of the mitigation efforts (Eg, 6 feet distancing, masks). The other thing I would caution against is the idea that people that go out with a fever aren't smart. People can run a fever without realizing it. I went to work with a bad headache this morning. Was it caused by a fever? No, but I didn't know that until I took my temperature (required before I go to work). A big part of the confusion is the term "asymptomatic" is ridiculously imprecise, almost to the point of uselessness from a disease tracking point of view. If someone runs a fever, but doesn't realize it or doesn't report it to a healthcare provider are they asymptomatic? Of course not, but they will be recorded as asymptomatic. A term that's generally used often to avoid that problem is "sub-clinical", where someone may or may not have symptoms, but the symptoms are not severe enough to cause the individual to engage with healthcare. "Asymptomatic" has been used to mean "sub-clinical" throughout the pandemic, even by people that should know better than to confuse the two. I assume that's intentional to avoid public confusion, but I think it actually making things more confusing.