I never said they did. Odd that you should make that argument.
However, as a US citizen myself, and a taxpayer, I believe I may have some say in the matter of how the government should allocate its resources. A law that has no positive effects that you can point to isn't worth paying to enforce.
Moreover, a law that penalizes harmless activity is likely to be itself harmful, in my opinion.
RE: 'Illegal immigrant' is a screwed up tautology