It's because they don't often have enough experience in life to have good judgement. Translated, teenagers are known to make bad decisions more often that older people. Their car insurance rates relative to other age groups are one small shred of evidence of this fact.
Not that I'm not elated that they did this, but why would any thinking person put their plans for shooting up their school on Myspace? Did they just want to be helpful to the police and courts who will use that evidence to lock them up?
Sometimes I think that Myspace and Xanga and the like are the best things to happen to adults in a long time. It's like some teenagers are just turning their diaries over to us to read. How different would school life be today if Klebold and Harris had had Myspace accounts?