Don't they have to ask registration to show that the poll is not tilted to dems or reps? If all respondants were from one party the poll numbers would be skewed to the right or left and thus meaningless as far as the whole country goes.
You could acheive the same thing by asking partisan identification -- in fact, that's what most polls do. The reason they don't ask for registration is that the rules of registration vary widely from state to state, so it is not an equally valid measure of partisan loyalties in different parts of the country.
