Why does Django admin list_select_related not work in this case?

The issue here is that setting list_select_related = True just adds a basic select_related() onto the query, but that call does not by default follow ForeignKeys with null=True. So the answer is to define the queryset the changelist uses yourself, and specify the FK to follow:

class EventAdmin(admin.ModelAdmin):
    list_display = ('__str__', 'device')
    def queryset(self, request):
        return super(EventAdmin, self).queryset(request).select_related('device')

Since Django 1.6, list_select_related accepts a boolean, list or tuple with the names of the fields to include in the select_related() call. Hence you can now use:

class EventAdmin(admin.ModelAdmin):
    list_display = ('__str__', 'device')
    list_select_related = ['device']