Yeah this doesn't make much sense since they can make a ton of money at assemblies. They either know something big is about to hit them in the pocket book, scared something will, or maybe they can take this money and just rent a hall cheap enough and still make a killing.
Or maybe they have found that renting a hall and then inflating the expenses is easier to sell to the rank and file then trying to convince 1000 people to donate 10,000 in cash for one day in a building that's been paid for years ago.