Being born in the U.S. makes you a U.S. citizen, but not necessarily a natural born citizen. A natural born citizen requires US citizen parents at the time of birth. It is a matter of dual allegiance, which the Founding Fathers specifically put in the Constitution to prevent the following:
According to your contention that only birth in the U.S. is required, Anwar al-Awlaki is eligible to be President because he was born in New Mexico.
Al-Awlaki is the “spiritual leader” of al-Qaida in the Arabian peninsula, whose sermons at the Dar al-Hijrah Islamic Center in Falls Church, Virginia were attended by two hijackers who carried out the 9/11 attacks, Nawaf al-Hamzi and Hani Hanjour.
Likewise, you believe that Fort Hood shooter and al-Awlaki disciple Nidal Malik Hasan is eligible to be President because he was born in Virginia.
Being a bad person does not preclude you from being eligible for POTUS.