The argument would be that you can be covid positive for (I think) 5-7 days before testing positive. So, that 72 hour testing thing doesn't mean much. Some will tell you that asymptomatic spread isn't a thing. I will say that contact tracing has established that, in fact, positive/no symptom patients can spread the disease.
And, I don't disagree with you that this breeds mask cynics. There is still little understanding of this in the general public. As an example, I went to k-12 with a guy who passed away unvaccinated yesterday. Everyone in his household had tested positive except for his 10 year old daughter. Since she wasn't having symptoms they let her go stay with her 80+ year old unvaccinated grandparents.
I'm not suggesting that it's completely risk free. It's not. As you point out, tests aren't perfect, and they don't make children test. It's still possible to get Covid on such a plane. My point is that it's relatively safe - compared to other scenarios in which people aren't tested. I'm having a hard time coming up with common scenarios in public that are safer. If you would mask in such a situation, when wouldn't you mask?