I was once an academic, but these days I'm both in industry and science/maths popularisation. I try to attract people to my site to get them engaged with the material I write, and then try to keep them engaged and interested. Perhaps they will then ask me to come and speak for them.
When I write something and put it on my web site it's because I want people to visit. Their engagement with the site lets me get sponsorship to run the site, and potentially speaking engagements. Speaking engagements lose me money, because I take unpaid leave from my job in industry to do them, but I think enrichment in education is important, and I'm willing to give up money to help make it happen.
Think of it effectively as volunteer work.
If material is ripped off wholesale and stored somewhere else, or if essays/articles are shown on another site without the visitor coming to my site, I lose my traffic, potentially I lose my sponsorship, and certainly I lose my opportunities to provide enrichment in schools. Doing what scribd does undermines that.
Other people have other reasons for people to visit their sites, but mine alone mean that I hate what scribd does. I think it's unethical and parasitical.