Also, from his lawyer: "They didn't find pentagrams and candles. He wasn't listening to Judas Priest."
WTF does that have to do with anything? Do only Judas Priest fans shoot up kindergartens? Is there something wrong with liking pentagrams and candles? I think that lawyer is the real creep here, and that kind of defense deserves to be ineffective.
In any case, if you threaten to shoot up a kindergarten, of course the police is going to look into you. Imagine the outrage if they got a tip like that, ignored it, and then the guy shot up a kindergarten? They have no option but to detain him, and the lack of candles in his home is utterly irrelevant to that.
And of course they then need to gather complete evidence, which includes the facebook conversation, details of his past, his mental health, etc. And maybe they messed up there. They clearly messed up some other stuff, in particular the jail where someone can get sexually molested, and the fact that they conned him out of a lawyer. The article does list a number of worrying aspects of US law enforcement injustice that seems way too common lately. But that doesn't change the fact that if you make a death threat, you should expect to get arrested.