"Without a proprietary period, an astronomer with a brilliant insight might spend years developing it, months crafting a successful proposal to execute it, and precious hours of highly competitive JWST time to actually perform the observations—only to have someone else scoop up the data from a public archive and publish the result."
and
"Without a proprietary period during which the astronomers who proposed given observations have exclusive access to the data, those researchers will have to work very quickly in order to avoid being scooped. Receiving credit for discoveries is especially important for early-career astronomers looking to establish their credentials as they search for a permanent job."
This is the real issue - it's an incentive that works against what's ultimately best for society. The author of the proposal should be credited for any work that results from their proposal - and that should be counted when considering employing the researcher.
Knowing what to look for is an incredibly valuable skill, in pretty much every career.