First, I don’t think twinking is banned per se. But this week seems to mark a change in policy based on a ban wave of people bypassing the XP debuff of queueing with an XP-locked toon. I wrote up a detailed report of the XP debuff permaban if you’re interested in the details.
It all comes down to content scaling being a fundamentally bad design (IMHO). The only way to make this work is to make you weaker as you level. And if all content is scaled, what exactly is the point of levelling other than being an arbitrary time gate?
We saw this play out in an extreme way in MoP Remix where level 10-30 felt great and 60-70 was utterly miserable.