That's a different situation though. You're then distributing something intended to link with readline -- not just implementing its API yourself but using its entire original implementation.

What you could perhaps do is create your own implementation of the readline API distributed under a different license and then claim your program is intended to link against that, but how would you claim that if no such implementation exists? Whereas if you have to write your own readline implementation in order to use it then you still end up with a large incentive to avoid that by releasing your work under the GPL instead.