Now we see why it is not beneficial to invite the Brady Campaign Against Gun Violence to join us.
While most of us aren't interested in making this a political football, the Brady's will take this opportunity to make a media event out of something that shouldnt be an event at all and politicize the venue owners desire to simply make a dollar.
I do not recommend calling the venue owner, as this is diverting their attention away from their primary focus of running their business. If they want to back out of hosting, calling to urge them to do otherwise is probably going to make it worse. Cancelling the event will give the Brady's a percieved win. The business owner banning this activity will give the Brady's a genuine win that they will exploit. There should be a calculated withdrawl that will make the business owner some money while not being made a target of the anti-gunners.
The only thing that I can think of that might mend the business relationship is to have the meeting sans conspicuously armed people. If there are no obviously armed people present, there will be nothing for them to protest. You might want to turn this around and use this development to 'punk' the Brady protesters. Maybe someone can figure out some way to flummox the sign holders... while making the business some money.
Instead of organizing these mass events and announcing them to the general public, I would recommend keeping the numbers small and make the venue private, announced to only those who have RSVP'd. This way, the Brady's will not be able to contact the business owner to harass them into complying with their desires. Another thing to consider; the Brady campaign does not have the numbers to effectively protest multiple venues. I would suggest adopting a strategy of announcing one location while the membership actually meets somewhere else. Or having more than one meeting location on the same date and time, with membership spread across a territory.