Before being used as a Kingdom Hall, it is cleared of any relics of false worship. Being thus adjusted, it is dedicated to Jehovah for the sole purpose of worshiping him. There is no sharing or fellowship between true and false worship.
And I am sure that there just HAS to be some bible verse they can pull out of their ass to back this up. Maybe from the old testament where the jews went in to the "promised" land and did just that with all those temples to Ba'al and Dagon etc.!