Fake controversy. He's not paying anyone on his staff less than 15 bucks an hour, the problem is that some salaried staff are working more hours that they are not paid overtime for (like most salaried jobs in the world). His response is to make sure that they are only working the 40 hours a week for which they are actually paid, which is the entirely appropriate response.
The entire point of minimum wage legislation is that it only works if everyone has to pay it. The academic studies that show minimum wages don't result in net job loss are about the total society wide impacts - obviously they make certain low-wage positions no longer viable, but they increase consumer purchasing power which opens up other jobs.
Bernie's campaign is already at a competitive disadvantage because it isn't taking corporate bribes, so having to pay the minimum wage puts him at an even greater disadvantage. If the $15 dollar minimum wage law was in effect, then he would be less disadvantaged. That's the point.
Also, Imagine being the cunt who works for Bernie Sanders and then going to the media to try and rat fuck him over this. It's treacherous behavior. Political campaign work is not a regular job and many people do it for free. You are supposed to put in extra effort if you actually believe in the cause.