If I were a gun rights group I would have a tough time making the decision one way or another on this one.
It could be seen as sending a message by not attending that you do not agree with the ban on a certain style of semi-automatic firearm.
Staying involved could be used as a platform to say "We fully disagree with what the producers of this show have done, and here is how we can stop it from becoming law."
Regardless of their reasons, I hope that every business and organization that has decided to boycott takes full advantage of the money they paid for their booth space. I hope that everyone of them hangs a banner in the booth they already paid for that says "XXXXX has chosen to not participate in the Eastern Sports Show due to their support of the denial of the Constitutionally protected Rights of all law abiding citizens of this great nation."