Why does rebirth denial come into this
Perhaps because the Commentaries are quite clear about it, but a common argument that one hears is that the "serious Suttas" don't talk about literal rebirth.
Another example would be Anatta, where the Commentaries (as quoted in the Visuddhimagga) seem quite clear that you won't find a self anywhere. For example:
"For there is suffering, but none who suffers;
Doing exists although there is no doer;
Extinction is but no extinguished person;
Although there is a path, there is no goer."
Visuddhimagga, XVI, 90.
However, the Suttas never quite seem to deny a "self outside the khandas", something Thanissaro Bhikkhu, for example, makes a lot of with his "not-self strategy" idea that permeates his writing and translations:http://www.accesstoinsight.org/lib/auth ... tself.html
I do, of course, have a lot of respect for Ven Thanissaro. He's perhaps a good example of someone who has looked carefully and sincerely and rejected certain parts of the standard Theravada in favour of his readings and experience. However, whether or not one agrees with him, I think that it is important to realise that his translation on Access to Insight have his particular opinions embedded. And unlike Bhikkhu Bodhi he is not always clear where he differs from the Commentaries. Bhikkhu Bodhi is always at great pains to state: "The commentary says X, I disagree because of Y".