DUH. Once again, (listen up greenies!): Hudson's Bay is a body of water surrounded on three sides by land. Right, three sides by land. The open end is water (to the north), where it is likely to freeze, and commonly does so, especially in (but not limited to) the season known as winter.
The bears have been feasting off the garbage dumps outside Churchill, Manitoba, for as long as I can remember, and have been known to walk into town.
Humans, not entirely off the menu for polar bears, will be extinct before the polar bears disappear (in search of better hunting).