I think they have a set amount of 'worldwide work' donation, sort of a quota, that they consider a fixed expense. It has little or nothing to do with the expenses that are directly attributable to the event. They also are still having to maintain their assembly halls, which are virtually unused but still need minimal heating and cooling, etc., so it is likely that they are collecting for those expenses.
In this area (midwest US) they had the assembly hall for sale for several years but no one would buy it. It is located so far out in the rurals with poor access to highways that it is essentially an unsaleable property.