I will agree with you that there are also Republicans who do straight ticket voting.
This is a practice that should be banned and is only currently allowed currently in 14 states.
The reason to ban straight ticket voting has nothing to do with Democrat vs. Republican. The problem is that a large percentage of straight ticket voters fail to vote in non-party affiliated elections (e.g. for judges, etc.) and non-candidate initiatives (bond votes, state amendments, etc.) at the bottom of their ballot. Straight ticket voting causes a significant reduction in proper electoral representation.