I have already expressed my disbelief at the verdict in a Facebook post, but here's a bit more about why the acquittal of the trio who have organised the procession ostensibly to deliver letters to Istana is such a big deal.
First of all, let's establish the facts – they weren't charged with organising an illegal gathering as such (even though that would have been in order too) but rather a procession on the grounds of a prohibited area that extends beyond Istana's boundaries and includes public streets outside of its actual grounds.

Under section 15, the judge argued that prosecution had to prove that the organizers "knew or ought reasonably to have known" that holding such a procession was prohibited. And because it was a public path and there was no signage explicitly warning that it is not, then they could not have "reasonably known" they shouldn't do it – hence the acquittal.
However...