They have probably more important things to do. Besides venting frustrations by speaking one's mind can be a very useful safety valve.
Based on my experience of the US military - these folks are usually not at all like civilian spin doctors - they tend to speak their mind and most of them loathe fat cats and politicians. However, they also tend to have a deep sense of professionalism - they do what they do because it is their job rather than to achieve some political or ideological end.
Wojtek