Because they were considered outlaws, pirates didn't often advertise their presence somewhere ahead of time for their own safety. So for them to do just that meant that they had to have a really good reason. They wanted people to come and watch...although what they wanted them to watch was anyone's guess. Jonathan wasn't able to stay away. He just had to know what they wanted badly to risk getting into a fight with the navy. So he found himself at the place where the meeting was said to take place. 

It was a little overwhelming (but no really surprising) just how many people had arrived here. Jonathan couldn't remember the last time he had seen Loguetown so packed. It was hard to believe that the navy wasn't here to stop this yet. 'Then again...they might be after all. Who says that marines always have to wear those uniforms all the time. Without that outfit, they look just like everyone else. So who can say if any of them are really here or not' he thought to himself