An ethical license restricts the use of a package - that is its intention. The difficulty might be that someone using Elm packages might not realize they are pulling in an ethical license, say if they pull in a package as a transitive dependency.
But the same issue already exists with GPL, someone could pull in a package with that accidentally, not realizing that it requires them to provide the source of their entire application. So this means my point above is not really much of an objection to adding some ethical licenses to Elm - its still up to application authors to do their due diligence checks on licensing thoroughly.
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Allowing non-OSI licenses needs to be part of my private package tool, and is something that I did not address yet in the spec, so thanks for bringing this up: