Scope difference of alias + alias_method a bug or a feature?

ruby-talk
1

Chan Sammy in ruby-talk[284135] stumbled over the different scope of
alias and alias_method. See his example a little changed:

puts RUBY_VERSION #=> 1.8.6
class T
  def self.redefine
    p self
    alias_method :bar_alm, :foo
    alias bar_a foo
  end
  def foo ; puts "T's foo" ; end
end

class S < T
  def foo ; puts "S's foo" ; end
  redefine
end

s = S.new #=> S
s.bar_alm #=> S's foo
s.bar_a #=> T's foo
p S.instance_methods.grep(/foo|bar/) #=> ["foo", "bar_a", "bar_alm"]
p T.instance_methods.grep(/foo|bar/) #=> ["foo", "bar_a"]
__END__

So:
alias_method results in an instance method bar_alm in S.
alias results in an instance method bar_a in T (and so in S).

I would like to repeat his question to straighten it out:
Is this difference between alias and alias_method a bug or a feature?

Thanks
Dirk