Nomura said that crying = indication of having a Heart in regards to Roxas, IIRC. He kind of shot himself in the foot about that seeing how Axel and Namine cried too.
Now, Axel cried after he already vanished which let him be reunited with his Heartless, possibly, so that can explain that.
And Namine, well, no one knows what she even is so who's to say she doesn't have a Heart? Lol. And no, Axel saying he and Namine didn't have Hearts as opposed to Roxas doesn't necesarilly count, seeing how his knowledge about Namine would've came from the Vexen stuff he read while in Days and we don't know what was even there to judge its relevance.