So here's my best guess at what's going on based on what I've read:
If the City of Tampa government cancels WrestleMania, then WWE can likely recoup at least some of the costs associated with running a large-scale event through insurance
If WWE cancels, they cannot and are on the hook for all refunds
Since neither side has made a decision, some of the smaller indies that run shows around WM weekend literally cannot cancel their events without potentially going bankrupt