Your scenarios are not at all equal. The terrorists on 9/11 set out to do harm. If only we would have intercepted them.
Tweeting about a bomb at a school (even in a potentially joking manner) would fit the scenario you describe, whether intending to do harm, or a kid being dumb and not thinking through the ramifications. That is where you intercept a kid and possibly press charges.
Tweeting "@Pink I'm ready with my Bomb. Time to blowup #RodLaverArena B!*@h [my edit]” does not fit this basic premise. The artist (Pink) sings a song (Timebomb) that includes lyrics about bombs blowing up. It was a public message directed at her referencing her music. Any person paying money to attend her concert is likely at least familiar with the song, and would understand the reference. I don't think it is reasonable to suggest in any way that this could result in panic, danger, harm, etc. The statement "time to blowup X" or any iteration thereof is common in today's vernacular.
Given the circumstances above, I completely disagree that this statement could at all be interpreted as a "notice of intent", ESPECIALLY in the same line as someone stockpiling fertilizer. I would suggest there is a huge leap between the two.
There IS an alternative action going forward - save criminal charges for criminal acts. This is an overblown misunderstanding caused by overly sensitive persons who were unable to critically assess his statement about the arena in context of the planned event. Bomb - song reference. Blow [it] up - common phrasing: have an awesome time. Venue - concert location where Bombs will be on topic and Blowing up as a sentiment makes fine sense.
"grandpas 90th birthday party rn everybody cruise!!! blow it up!!!" - mass murder?
"You want to burn this bridge? I'll blow it up." - planned terrorist strike on key infrastructure?
"Just learned termites have weaseled their way into my home, so now I'm trying to decide if I wanna set it on fire or blow it up w/dynamite." - potential insurance fraud?
"I'm the most annoying texter. Like if you don't answer me I'll blow it up. Idgaf"\ - murder?
"2223 delaney ave 32806 Tomorrow's address blow it up!!!" - detailed murder plans?
"Building a bridge just to tie you to it and blow it up." - terrorism with named target?
"I don't wanna hear anyone annoyed tonight with fireworks pictures or tweets this is America baby blow it up" - terrorism?
"Dad is talking about how easy it would be to bringabomb onto a ferry " - terrorism?
"College Tip: You can get out of class early if you bringa smoke bomb or any bomb." - terrorism?
I do understand there is a difference - the kid's tweet called out a specific place during an event, and included both statements "bomb" and "blow it up". Simply looking to illustrate that these are not irregular or threatening statements, they are part of common vernacular, and assigning "notice of intent" seems somewhat unreasonable.