Cruz didn’t blame him for it, but he said the tone of Trump’s campaign encourages it. That is simply true from a psychological perspective. Rubio is the proof of that. Rubio would not have insulted Trump’s looks and other things if Trump hadn’t done the trash-talking first himself. When a campaign lowers the bar of discourse, they encourage more of it from their opponents. It doesn’t mean the opponent isn’t also responsible for their actions, but lowering the bar when it comes to insults or not condemning violence clearly is a contagious thing.
So, Trump didn’t cause the violence, he only created the environment that caused it.
Sorry - I do not see it reasonable to interpret the statement this way in light of the active political ambitions of BLM, etc., and the fact that Bernie Sanders supporters are publicly bragging about their roles in perpetrating the act.
IMO, Cruz essentially said that free speech does not apply unless nobody finds it offensive. That is not, and cannot be, free speech.