It's controversial for the same reason that nearly every Facebook political meme is controversial: It takes a quote out of context and implies that there was a much different meaning than what was intended.
The snippet that is being quoted on Facebook is meant to imply that Bernie Sanders doesn't believe white people can be poor.
However, if you listen to the entire quote, he's clearly talking very specifically about poor black urban youth.
Yes, white people can certainly be poor, and grow up in neighborhoods if not called ghettos, certainly akin to them. But white people don't typically have to deal with the entire list of issues that Bernie Sanders pointed out...namely the constant harassment one has to deal with due to skin color.
It's controversial because people like to react to things sans context
To be fair, this is how politics often works...taking quotes out of context and paint the opponent in a much different light than was intended by the full quote.
If I were running for office I might say "I don't like children being left behind by society" but you can bet there's be a Facebook Meme of me quoted as saying "I don't like children" :)