It's Cassandra whom I'd most expect to do something about it, actually. As a Seeker, templars were her charges in a way, and if memory serves she even mentions making things better for templars who want to leave when she speaks about what she would do as Divine. But, yes, it's a shame and a complete oversight by the writers.
Well maybe she tried to help Cullen but couldn't. Just maybe he was too far gone.
"When he was thirteen years old, a visiting knight-captain took notice of Cullen's hard work and offered him the chance of formal training. His parents agreed to the request, as the Templar Order was known to be a respectable and honorable profession, and they wanted to give their son the opportunity to have a life he would be happy with."
Let's remember Cullen joined the Templars at 13 because he wanted to help people stay safe. He was one of many boys who joined. Do you think these children were told the truth? The Chantry/Templar order was the first to betray him, the mages tortured him, Meredith his boss went postal on him, and his new boss and friend pushed him to take more drugs. Seems to me this is not just an issue of drugs, but also of betrayal and not by just one source.
If the goal was originally to do good, and keep people safe working in a respectable and honorable profession, then everything he reached for was a lie. Pushing drugs on someone is doing your best to destroy them. Now Cullen sees a world where he can't trust anyone or anything. Templars/Chantry and too of his bosses totally destroyed him.