Those in the US without documentation (a valid visa, green card, citizenship, etc.) are not criminals. The vast majority are folks who overstayed their visas. Doing so is not a crime. It's a civil tort. That's the law in the US.
Why are you advocating breaking the law by treating non-criminals like criminals?
Correction: They're building more prisons specifically to fill with ILLEGAL immigrants. The vast majority of whom have committed crimes while here.
If you have a problem with criminals being put in jail then you have a much bigger problem with your moral compass.