Why do some soundtracks default to non-lossless when they're available??

[NMErickson]

As soon as I popped in my new blu ray, Natural Born Killers today, the Dolby Digital 5.1 soundtrack came up as the default. The disc clearly supports Dolby TrueHD 5.1. WHY DOESNT TRUEHD COME UP AS THE DEFAULT?? For some of my blu rays that have it, TrueHD comes up first and theres no need to change the sound setting. But for others, notably Blade Runner, 300, and V for Vendetta, TrueHD is there, I just have to change it manually from Dolby Digital. Is this normal for these discs or do I need to change the settings on my receiver?