Most of the polling and election methods that have a "Vote for only one" rule use the defective "vote for only one" Plurality Voting System. Whenever more than two candidates run, it unfairly favors the candidate most different from the others.
All fair voting systems must collect the votes on each candidate separately from the other candidates. The voter is not prevented form voting for or against any candidate. To be fair, a voting system must let voters prefer more than one candidate.
Any newscast that gives percentages of the vote for candidates is getting these figures from polls using unfair methods. Any polls that use "Vote for only one" rules are using unfair methods. These polls match the election results only because the elections are also using the same unfair methods.
More can be found at Unfair and Fair Elections: the Menu