This is in fact a very good feature. When power is cheap or negative it's because there is more generation than load in some segment of the grid. To fix this you can tell people to stop generating, you can tell people to start loading, or you can just drop the price and let both happen on their own.
There are targetted ways this is done when the difference is so big it threatens grid stability, but its often better to let the market handle the gross imbalance on longer timescales when grid stability is not threatened.
There are some things markets are very good at. This is one of them.
