I'm not buying your reason why the ad was made. The obvious purpose of the ad was to portray Trump as anti-flag and appeal to the confederate flag supporters that Ted stands with them. If Cruz supported the removal of the flag, why make the ad? Unless he is using the issue as a way to garner votes (pandering). And if he is upset that Trump "got involved" in a state issue by expressing his opinion as a private citizen, he is not that good of a constitutionalist.
First thing, Cruz didn’t make the ad.
But, yes, a private citizen has the right to express an opinion on a given subject. That does NOT insulate said private citizen for negative response to his opinion. This freedom to speak one’s mind goes both ways. So whoever made the ad is well within their rights to criticize Trump for his position, don’t you think?