count works differently than find with polymorphic include

It looks like AR::Base#count has different behavior than AR::Base#find when a polymorphic association is listed in the :include option. For instance, assuming with we have a class Foo, with a polymporphic association "bar", I see the following behavior:

Foo.find(:all, :include => :bar) # works fine
Foo.count(:include => :bar) # raises ActiveRecord::EagerLoadPolymorphicError

I see that ActiveRecord::Calculations::ClassMethods#count is documented to use an outer join for associations in the :include, but this feels like it is out of date in light of the changes made to the way #find makes use of the :include hash. We can already use :join in #count, so why leave the path through the :include cause as well? It feels weird that the values that are legal in the :include hash for #find are not necessarily allowed in #count. Was this a backwards compatibility issue? Maybe I'm missing something?