Being liberal means never to have to say you are sorry. It's all about the unions, I bet. I have not seen the fine print, but, I bet the a unionized shop gets a break on the $15 / hour.
Even worse, many union jobs are paid a multiple of the minimum wage; for example, a city contract may require workers on a project, in addition to being 51% minority/women, are paid 125% of the minimum wage - THAT is why the unions want it. The fact that wages killed so many jobs is meaningless to them...