I believe that there is a second and more dark intention in this proposal. Today Switzerland is part of EU but they still have quotas for "work permits" depending on the nationality of the worker, this goes against the principles of Schengen (
https://en.wikipedia.org/wiki/Schengen_Agreement) and there is lot of pressure for Switzerland to remove these "quotas" (mainly against eastern European countries). The problem is that the wages there are way higher than other Schengen countries, and without the quotas there would be massive immigration to Switzerland that would lower sharply the wages for jobs that don't require higher education. So, IMO, Switzerland wants to give a minimum wage to create a "virtual block" to the poor countries of Schengen... (I am French and I live near the border)
edit to clarify: if you are a Swiss citizen you would earn your wage + state wage, and if you are an immigrant you would earn just your wage... making virtually impossible to afford living there (considering your wage =~ state given wage), only for very well paid jobs that this would make no difference.
edit: typo