Under the heading of "Places not to be"

would be 'Anywhere assorted biker gangs are gathered'.  Especially if said gangs feature names like Bandidos and Cossacks.

Note I said 'biker GANGS', not groups or gatherings; been to/around a number of those, no problems.

From another story:

Swanton said that the restaurant's operators also were aware of the meeting in advance, and he described the management as uncooperative with authorities in addressing concerns.

"Apparently the management (of Twin Peaks) wanted them here and so we didn't have any say-so on whether they could be here or not," Swanton said.

So I'm guessing various members had been at the place before, there'd been problems, and when they found out a BUNCH of each would be there they asked the place to tell them no meetup.  Pure guessing: despite previous problems the manager figured "They spend a lot of money and tip big, so who cares?"  Well, you do now.