Because we do actually need evidence and **** to deport people. The family that's been here for 20 years "with no problems" may very well have some problems (fraudulent documents, obtaining documents via false instruments, etc.). Those guys at Home Depot have no documentation whatsoever and no fingerprints on file (or, better yet, they do have documents...that they bought from some guy who stole them out of a filing cabinet in Puerto Rico). If you can't positively ID them as illegal immigrants, you can't deport them.