That was not a decision made by the president. A decision to fly flags at half staff for prominent residents of specific states is made by the governor of the state. The flags were half staff only in New Jersey for Whitney Houston's death, a decision made by Chris Christie. GOV. PERRY, of Texas did not make the decision to so honor Chris Kyle. You can read about this in Title 4 of the united States Code Chapter 1 Section 7 http://en.wikipedia.org/wiki/Title_4_of_the_United_States_Code
Flag Etiquette ✯ STANDARDS of RESPECT ✯ The Flag Code, which formalizes and unifies the traditional ways in which we give respect to the flag, also contains specific instructions on how the flag is not to be used.