The house where Hollow Earth Radio is located is hardly a DIY-show space. This gathering was truly a regular BBQ potluck where two low key bands happened to be playing in the living room. There wasn't any illegal alcohol sales or consumption, and combined with the relative low volume of the two acts and no entry price, if the police had shown up they would have had no premise to go from. But they would have been welcome to grab a plate like @2 said.
The house's address was added to the poster to invite the public to this community-oriented event. Because of the aforementioned lack of illegal activity, there was no fear in posting the address publicly.
Thanks all for the concerns on Line Out comments, though! And to Gina Y for the kind words.
--The actual Hollow Earth Radio Spokesperson