Perhaps those reasonable people are making a moral judgement when they decide to use proprietary code in a certain circumstance: they're deciding it's morally acceptable.

And maybe it isn't morally acceptable in some circumstances. Like when transparency is required in order to facilitate accountability and peace of mind.

It's immoral to have closed-source voting machines. But is it immoral to rely on closed-source search algorithms? The answer to that is less obvious. Either way, moral issues affect code all the time.