I agree with you though that it would be great if local police were enabled to find and deport all of the illegal aliens currently residing in the US illegally.
Why? It is a misdemeanor.
You've got no legal right to park in a handicapped spot, either, but that doesn't make it a felony.
The only reason to point it out as a misdemeanor is to make it seem like it is not a problem.
The reason for this separation has to do with consistency and scope: legality of immigration, and illegality of counterfeiting, should be consistently the same from state to state. Cases involving these issues could happen in any state and are likely to involve more than one state at a time, whereas most police work usually involves only a small area. They also are not areas where the states have authority to set policy, so it could create difficult situation for police if there were a disagreement between their state and the federal government over an issue like income tax. These factors together seem to explain why we have so many federal police forces.
How would things be better if local police had authority to detain and deport people, or to investigate customs matters?
- https://www.ice.gov/news/releases/police-officer-arrested-child-pornography-charges
- https://www.ice.gov/news/releases/arms-trafficker-convicted-anti-aircraft-missiles-scheme-and-other-arms-offenses
- https://www.ice.gov/news/releases/rwandan-man-sentenced-immigration-fraud-and-perjury-connection-1994-genocide
- https://www.ice.gov/news/releases/operation-cookout-indictment-charges-39-defendants-heroin-and-fentanyl-trafficking
- https://www.ice.gov/news/releases/former-fresno-resident-charged-illegally-brokering-sale-military-arms-foreign
- https://www.ice.gov/news/releases/ice-hsi-investigation-leads-federal-indictment-alleging-scheme-avoid-payment-18
(edited to fix formatting)