Genetically Polar Bears decended from Grizzly Bears. So with less ice the Polar Bears are spending more time hunting on land. That means more interaction with Grizzly bears during the breeding season. It's no differant than a Collie cross breeding with a Lab, or an Arab crossing with a Quarter Horse. It's really nothing new, just seen more because of current environmental conditions, and there is more people going to the remote palces, more than before. Makes for genic diversafication.
And when you hear some environmental whacko say Polar Bears can not live on land they have to be on the ice to hunt and survive. That is BS! We had a man kill a Poler Bear last year here in Alaska over 250 miles from the Arctic Ocean. This bear was healthy, and had been living on land for some time to travel that far.