Reagan was the one who started this crap - no president before Reagan ever saluted, including those who had held high military rank. Every president after Reagan has done so - presumably, after Reagan and Bush Sr. had both done it, the political analysts most likely decided that ceasing to do so would be incorrectly interpreted as a sign of disrespect to the military.
I'm far more annoyed with Bush Jr. over it in light of the combination of his own dereliction of duty with his "career" in the military, the fact that - as someone who WAS in fact commissioned as an officer, however lightly he took it - he should damned well know better, and for his blatant attempt to cast himself as a great war leader.
Even EISENHOWER distanced himself - as completely as he was able - from his military standing when serving as President, in keeping with the primacy of civilian law over the military.
Grrr. I'll stop now. But man oh man, this pisses me off.