If a bear is going to travel, it's going to travel. What man made impediments stand between Yellowstone & Glacier anyway?
Also, deer and elk migrate out of the Park each year. Deer have been documented migrating from I-80 near Rock Springs to Island Park, ID. And that's not why the elk feedgrounds exist or continue to exist. At this point, I think that's actually more political than a biological need for the elk to survive.