How much did Giles actually know about Willow addiction? Tara talks to her about and they decided on Willow's timeout in 'Tebula Rasa', but did Giles know as much as Tara did? I know he talks to her about how dangerous it was to resurrect Buffy, but I don't think it ever occurred to him that Willow had some kind of addiction, and, as far as my memory serves, he never talks to her about using too much magic. He was more concerned about her using such a dark spell. Giles comes back Sunnydale when he realizes how off the deep end Willow has gone, but I can't ever remember him mentioning anything before that.
On the other hand, as bad as it makes Giles look, I just don't think he cared enough about Willow. She was never his charge or responsibility, she was just someone who came along with the Buffy package.
When Giles left, his reasoning was that he wanted Buffy to stand on her own two feet and deal with her problems instead of hiding from them. Giles thought that having him around was hurting Buffy and giving her an easy out and escape from dealing with certain things, like Dawn in 'All the Way'. He thought that if he left, Buffy would pick herself up and be stronger for it. It was complete crap and a ridiculous idea, but there it is.
Giles' reasoning never included Willow or Xander or anyone else. Buffy is his main priority and he adopted a 'tough love' approach with her. Willow never factored into his decision mainly because she never has before. He's never had to consider that he should be worried about Willow and Xander, too. Not to the extent he worried about Buffy.
I'm sure Giles cared about Willow as a friend, but he just didn't care enough to consider the fact that she might need help. It's shitty and it makes Giles look really bad and it makes me hate S6 even more than I already do.