I think a lot of the premature calling has to do with wanting to be first, rather than wanting to help one candidate. And, as Beet pointed out, it's hard to say if either side is favoured by someone being named winner too early. I know that in Sweden, whichever side is leading in polls is usually carefully making sure to spin it as a close race to get people tp the polls.
And since stations and the AP get their information usually from VNS and exit polls, there is no bias, just the desire to be first. All media stations this year will not be broadcasting their "projections" until after all polls have closed for that state, so it's pretty safe to say we won't have problems with people being turned off from going to the polls.
