Because if a topless protester shows up, Sanders lets her in, and Trump kicks her out:

Democratic presidential hopeful Bernie Sanders this week joked about a topless woman who showed up at one of his rallies recently to protest Republican rival Donald Trump. 

“Well, I was trying very hard to get every detail,” the Vermont senator joked during an interview with Phoenix-area radio station KTAR when asked if he could read the text on the woman’s chest.
“No, just kidding,” Sanders chuckled as the male radio hosts erupted into laughter.
“No, actually I was trying to focus on a couple of other things,” Sanders continued, trying to stifle another laugh as he pivoted to discuss campaign issues of income inequality.
A woman who tried unsuccessfully to get into one of Trump’s rallies in Arizona over the weekend opted to show up at Sanders’s rally in Phoenix on Saturday instead.
The words “stop fascism” were revealed on her back and “hate speech is not free speech” were marked on her chest, as she stood on someone’s back, interrupting Sanders’s rally.

She was also wearing “Free the Nipple” pasties.

Photos (she is quite fetching) available at the links.