Failure to do so would enable a voter to both cast an extra vote for and an extra vote against, allowing voter fraud; by design, a for vote cannot be distinguished from an against vote once cast, so this multiple-vote fraud could not be detected until the final tally verification (and maybe not even then), and it cannot be corrected at that point or even traced to a specific voter.
If voters are allowed to cast both for and against votes without restriction, it would enable fraudulent voting.  Because the votes are indistinguishable once cast, this type of fraud can only be detected after the final tally, and cannot be corrected or traced back to the individual voter.