Uh, because his JOB is to veto unconstitutional legislation, regardless of which party passes it.
Where ever did you get such an odd notion? That is certainly not in the President's job description. It is not the Presidents responsibility to even determine if legislation is Constitutional. That is the job of the SCOTUS.
On a presentational note...starting a sentence with "Uh" does not make you sound smarter, just insolent. Perhaps that is the tone you were trying to convey.