More then likely teen jerks with nothing better to do.
The fire was extinguished and it appears to have caused little exterior damage.
The inside will need to be cleaned and repaired (tax payers will foot the bill)
I think the PD should offer a $500 reward for anyone with information leading to arrest and conviction. Let the idiot(s) who started it, pay for it.
BTW- This was after 8:00PM last night...why was the bathroom still open?