There's no difference between spending hours browsing the web and spending hours reading an eBook.
Wrong.
Browsing the web needs a.) a network connection b.) a colour display (otherwise you cannot really use a lot of the websites) c.) halfway decent CPU/RAM to handle all the javascript/graphics/animated stuff/...
Reading text (ebook, ascii file) needs none of that. The text is already on the ebook reader, so you need no network connection. You need no colour display, because it's just text, black on white/grey. Which means the e-ink display is perfect for the job, and it also uses far less battery power than a backlit display. And, in my opionion at least, is FAR easier on the eyes. If I had the choice of using the latest and greatest ipad or my kindle to read a book, I'd still choose the kindle even though the ipad has a really good colour display - the e-ink display is just better for text. And you can get away with far less CPU grunt because all the reader needs to do is refresh the display when you press the "next page" button, and text files use no java script or any other stuff websites use.
So, really, reading websites is not the same as reading an ebook.