There are thousands of high school students who won't be able to afford college if they are forced to pay the costlier out-of-state tuition. It's unfair to deny so many people access to affordable tuition. Many live and work in the state without documentation for years because the legalization process for residency is lengthy.
Some undocumented students will just drop out of high school if they know they can't get into college because of high costs. If undocumented students can't get into college, they can't serve as models (of academic achievement).
Some Californians don't want the undocumented to get equal access to higher education because they don't want immigrants to live here. (But) undocumented residents will stay in California even if they can't get in-state tuition.
The state doesn't benefit when people can't get access to higher education.