I've never heard anyone say they're only sending those kinds over. It's mainly that if you have open borders and let people in Willy nilly nothing will stop them from getting here.
“When Mexico sends its people, they’re not sending their best. They’re not sending you. They’re not sending you. They’re sending people that have lots of problems, and they’re bringing those problems with us. They’re bringing drugs. They’re bringing crime. They’re rapists. And some, I assume, are good people.” -Donald Trump, June 2015
He also assured that there are good people, they just can't come in illegally.
Checkmate libshits.