Des Browne says book launch is official engagement.

I can't understand why Des Browne considers attending the launch for Stephen Walker’s book, “Shot at Dawn” to be an official engagement. The book tells the story of how twenty-eight Irish men serving in British regiments met their deaths. Even though there is a military connection, I can't see why our Defence Secretary should officially attend the book launch of a BBC journalist?

I might be wrong, it could be quite a normal for a senior minister to carry out such duties, but if that is the case, I find it very strange. Maybe someone could enlighten me?