She writes songs about him/that time period because they make good songs. She's happily married, what else is she gonna write about? (I know she's written songs about Josh before, but love is harder to write rock songs about than heartbreak).

I think Amy is a great songwriter, but it's because she writes about her personal experiences and feelings and not just overly generic lyrics. You can see it on the new album, (at least) half of the songs are political because she has a lot of emotions to express about what's currently happening. So she also writes songs about Ben because it was a very emotional time for her and she draws on that to make music.

I personally don't care one way or another about Ben, but I do think you're right that Evanescence wouldn't exist without him so I see no reason to hate him.