Why are today's conservatives dead set against Reagan's immigration policy?

[Tinark]

Roland Reagan's Immigration Reform and Control Act of 1986:

legalized illegal immigrants who entered the United States before January 1, 1982 and had resided there continuously with the penalty of a fine, back taxes due, and admission of guilt; candidates were required to prove that they were not guilty of crimes, that they were in the country before January 1, 1982, and that they possessed minimal knowledge about U.S. history, government, and the English language.

http://en.wikipedia.org/wiki/Immigration_Reform_and_Control_Act_of_1986

Ok, what's wrong with that?

How is anyone helped by having these people remain illegal for their entire lives while living in the united states?