They did occur, but yes, at a very low rate. The rate will increase once the people with guns realize the benefit of such attacks. This is not the case now, but it will be the case if the price continues to rise.
Imagine that 2 court orders (one to GHash, one to the next largest pool) are enough to reverse a transaction, or to "freeze funds" by not allowing TXs from a certain key.
If the incentive system encouraged independent mining, this would not be possible.
Imagine that 2 court orders (one to GHash, one to the next largest pool) are enough to reverse a transaction, or to "freeze funds" by not allowing TXs from a certain key.
If the incentive system encouraged independent mining, this would not be possible.