He is trying to keep within the law to prevent someone seeking to clog up the courts system with a challenge for the sake of a challenge.
The flights will dry up quickly as you state PLUS countrys they leaving from may impose quarantine on US citizens leaving or airlines demand results of tests. No doubt some would be on to Embassy / Congress etc demanding release and a "formal" protest made while behind the scenes an informal if infected then we have no objections will apply.
There are other ways to quarantine people returning from abroad into the US with resorting to Presidential dictat.