Why would a voter who wants to cast a vote for a legal candidate not have standing?
To have standing, you have to be able to articulate an injury that you will suffer that is distinct from a generalized claim of injury to the public at large. In other words, you have to be able to show how you are injured in a way that is different from the general public.
Actually, you are right... the rules on standing apply only to Obama. Cruz will most likely will get a different type of treatment in the courts and in the media. This may go somewhere.
The courts reason that your tentative vote can't be viewed as having any value, that the person you object to might lose anyway, in which case, you have no damages at all.
The better line of attack in these suits is ballot integrity. It is a crime in all states to file falsely. There is a good argument that Cruz has broken the law by certifying that he is eligible.