I think it's on purpose (sorry if I'm repeating what other people have said) but it has repeatedly been shown, and said, that Buffy doesn't listen to Giles and that Willow doesn't listen to Tara (or Giles). OK, they do sometimes about some things of course, but Buffy has been pretty oblivious of Giles' state of mind going all the way back to at least Prophecy Girl - how do you get 'uncaring' from his quite obvious distress? Even if you don't know why he's acting weird how do you not notice and ask why? More recently, when Buffy asked him to deal with Dawn (S5), he says (I paraphrase) 'why would she listen to me you never have'. Tara (and Giles) have tried many times to talk to Willow and try to stop her doing magic. With Giles it goes back to Becoming 1 all the way up to Flooded, and Tara more recently (can't remember the exact episodes but there are many) has persistently talked to her about it. With both of them Willow has either played cute, avoided the question, outright lied, stolen books to avoid a confrontation, got defensive or got aggressive. She's never actually listened.
I hate that they both leave in the next episode but I can see why, especially for Tara. Tara is doing it to protect both her physical and mental health, Willow is frightening her. Giles, I think, is leaving for his own mental health and using the 'it's for Buffy' as an excuse (although he's not wrong just now is not the time). For him it's not just what's happened recently, it's the accumulation of losing so many people, and Buffy twice already. It's pretty much the first selfish thing they've ever done (barring that spell Tara did in Family and maybe other people can think of something for Giles, I dunno), and it is TERRIBLE timing, but I don't really blame either one of them (Tara, not at all, and Giles mostly not because he probably should have stayed, and knows it, but he can't).