In his response to the question, Dr. Hatem Al-Haj, dean of the Shari`ah Academy of America, stated,

When Omar asked the Prophet (blessings and peace be upon him) about the reason of praying qasr (shortening the Prayer) after Allah gave safety to the believers, the Prophet (blessings and peace be upon him) told him, “It is a charity Allah had given you, so accept His charity.”
Based on this hadith, the Zahiris hold the position that qasr is mandatory. However, according to other scholars, it is preferable, and that is the correct position. Thus, it is recommended that a Muslim may shorten the prayer, but if he completes it, there is nothing wrong in doing so.