The Witnesses make a farce out of their tax exempt status. The law presupposes that religions are akin to charities. Therefore, society should have an interest in having religions flourish so they can look after the poor and perform other good works.
The Witnesses don't do any of this. Their literature states that charitable works in general are a waste of time because their time is better spent on preaching. To the extent that they supposedly perform charitable works for their own members, such as repairing homes after natural disasters, it comes with the implied promise that the homeowner will donate their insurance check in full. I can't think of any officially-sanctioned charitable activities that are carried out without any strings attached.
Their entire outlook and approach runs counter to the purpose of having a tax-exempt status for religion in the first place.