Because I think that the law and precedent is clear, and the only questionis whether there are 1, 2, 3, 4, or 5 justices who are corrupt enough, and partisan enough, to vote to strike down the act.
If Kennedy votes not to strike down, I think that he takes Roberts with him, giving a 6-3 decision, but if he swings the other way, so does Roberts.
The Chief Justice will not be on the dissenting side of this vote, because of the optics, not because of the law.
Roberts may be a corrupt partisan hack, but he only uses the secret sauce when it makes a difference in the final decision.
My guess is that it will be 4-5 to strike down much of the law, but if not, it will be 6-3 supporting it.
What must be noted though is that this is not a matter of law. The law has been settled for at least 60 years.
The only question is how corrupt the 5 right wing justices are.