Cruz was making a statement, and expressing his opinion as an individual, as did other Republicans who boycotted the convention entirely.
Contrast that to the Democrats: none of them would DARE do anything like that. Herd/collectivist/scripted/Party-Line mentality all the way, no matter how big the lies or the crookedness.
Even Bernie Sanders was made to understood that, one way or another; he was sweating profusely when he gave his reluctant endorsement of The Venal Lady.