This is really all about lawyers and litigation. In a sane world, you would have every right to invite the audience on stage at your own risk and the audience would understand that going on stage was at their own risk and so on. The venue would have nothing to do with it as they were only renting you the room.
Actually, I really think that most people understand this but lawyers are unscrupulous bandits who profit from the misfortunes of others and make normal human interaction fraught with difficulties. So what ought to have been a celebration got turned into a scuffle because the venue were scared of getting sued. Very sad.
Well, it's not just lawyers who are unscrupulous. It is also people who hire lawyers. That is, some of the members of the audience. It's like guns. Guns don't kill people , people kill people.
And a lawyer is a gun in this case. Lawyers don't sue people for a quick buck, people hire lawyers to help them sue other people for a quick buck. People take no responsibility for their own actions and so, always look for someone to blame.
that being said, it doesn't seem very safe to invite the audience onto a stage that may or may not support their weight.