Remember when managing Threat was a gigantic part of the DPS' job? I mean, gigantic. People would choose trinkets and enchants just for threat reduction. They'd request a Paladin blessing that reduced Threat generation over ones that increased actual damage output, because you could do the most additional damage just by not being Threat-capped. A writeup like yours in TBC would have listed "Responsibly controls Threat output" as part of DPS' job description.
That changed and got overtly folded into the Tank job and largely taken off DPS' hands. Roles modify and evolve, and arguing that you don't want to do something because it's not the standardized manual-written procedure is a little nearsighted.
We've gotten used to the roles being very rigid and myopic, especially coming out of WOTLK, where everything was highly exaggerated and specialized (largely a chain-reaction from heals basically being limited only by activation time). The devs seem comfortable making the roles a little more fluid going forward, so I'd expect more boundary-blurring as WoW moves forward.