E-Readers and Tablets --Why are they Grouped?

I've read a lot of articles online comparing e-readers with tablets and I'm not quite sure why two separate things with similar form factors have become grouped together so closely.  The fact that Amazon released a Kindle that is a tablet hasn't helped the matter much.
I got a Kindle for Christmas last year and absolutely love it.  I preferred a Kindle to an iPad because I wanted it to read books, not to play games, surf, and get email.  I ended up acquiring an iPad a few months ago, and it's great for angry birds, surfing, magazines, and other things that aren't reading.  As far as reading is concerned, the iPad is okay, but I don't think the screen is made for reading for more than 30 minutes or so at a time.  I have used it to read, but only because I left my Kindle downstairs and was too lazy to get it before bed.  With a Kindle (or e-reader) you forget you're using an a device instead of a book, it's hard to forget with a big clunky tablet.
Bottom line, if you want to read books, get an e-reader.  If you want something that's great for casual gaming (think angry birds), magazines, or something light to surf, get a tablet.  If you want both, save up and fork out the cash for both, but don't get a tablet for the soul purpose of reading books, or even with that as an intent.  I can't speak for who the Kindle Fire will work but I just can't see why these items have been so closely grouped together. It'd be like getting an iPad to replace your iPod or your camera... yes it does the same functions, but you look just plain silly out in public taking pictures with an iPad or trying to jog with one.

Note:  I wrote this for two reasons, I get asked frequently "Should I buy a tablet or an e-reader?"  Another reason, is I read this really great article the other day called "Why Did I Buy A Kindle 3 When I Already Had An iPad?" by a blogger/podcaster whom I follow.  His conclusions were much the same advice I have been giving for a while now.