Perhaps you didn't read the article. The vendors can't just show up and do what they'd planned to do at this event. The event organizers changed the rules of the event to bar so-called 'assault rifles', etc.
Those vendors now backing out, are doing so because they either manufacture or sell the aforementioned 'assault rifles'. Those rifles are also their biggest drawing items, but now they're not being allowed to display or sell them at this event.
This is a business decision on their part. They're going to lose income because of the event organizer's decision. These vendors also happen to fully support the 2nd Amendment, which is the moral side to their protest.
I stand corrected.