Critasism must be leveled at both, and the only criticism that means anything to a party that matters is “I will not vote for them” regardless of the truthfulness of that statement. The only influence we have over the government is who we vote for.

To say you would vote for one party no matter what means you have no influence over their platform and tacitly accept everything they do, meaning you have basically given up any influence over the government in the same way as if you never voted at all. Demanding that others do that is insisting that people not use the one influence they have over the government.