that's because it's considered a rare one. You can only release commons (to save people making an accidental freeing perhaps). It wouldn't allow you to release a rook/rube/magic rock, gwen, helga etc either.
Thanks Arabesque!
I was wondering about the new achievements, but your answer made me read ithem again and alas...it says: Set free one each of the releasable Series 1 Cubimals