Wouldn't it be nice if life was black and white.
Everyone is good -- or evil. All issues had right-or-wrong solutions. Sort of like in Super Hero movies, for 7-year-olds.
Many of these Dreamers were brought to the U.S. as 7-year-olds, or even younger. 
Unfortunately, life is not like that and these people are caught in a unfair legal limbo.You are suggesting a child should be held legally responsible for the actions of his parents? 
Many of these people are now young adults contributing to economic and cultural health of the U.S.
Hundreds of CEOs from companies,including Apple, Mircrosoft, Amazon, GM, have urged Trump not to end the Dreamers program.
"Illeglal is illegal"  is out of a cartoon moral universe.
These people are caught in real life.