What forces them to insure is being a good operator: insurance is risk management. If you're someone like Sears Canada, for example, I suspect you don't have insurance because you're big enough to look after it yourself.
Those of us who have insurance on our cars and our homes and our boats have it because if we have an accident we can't simply write a cheque right there and then, and we ask an insurance company to do it for us. Good operators with a bottomless bank account don't need insurance. They can look after any claims they're presented with themselves; they can self-insure. But what you find is that most operations have cashflow issues, so they don't have bottomless pits of money. From a risk management point of view, they spread their risk around, saying that if A happens, then this insurance company will look after it, and if B happens, then that insurance company will look after it.
I think the whole idea of compulsory or not compulsory insurance is getting away a little from the true focus, which is on having a minimum safety standard and making sure the good operator is running a good tourism operation.