Why did John McCain have to support amnesty for illegals?

I liked John McCain up until he supported amnesty for illegals. He is right about how we need to stay in Iraq and not cut and run, and I did not support the war when it began, but we must stay there. John McCain was a war hero who served his country bravely and was held as a POW for an amount of time, and came back to the US as a hero to everyone besides the hippies who hated our troops. I supported John McCain up until this year, because his being held as a prisoner of war means he can deal with a lot of pressure ans stress, and I believe he is a pretty honest and decent man. I can't believe he supported the amnesty bill. He still has my respect, but he no longer has my vote.