I get what you mean, and I buy books on amazon for my kindle pw

And that's my point :
When you buy books from the same store as your reader brand, you get a lot of reading features, which doesn't work with sideloaded books.
With ebook from AmazonStore read on your kindle, you have :
- sync between devices
- freetime
- sharing quote on social network
- good reads
And with ebook from KoboStore read on your kobo, you have :
- sync betwin devices
- extend stats reading
- and maybe others features that i'm not aware of (I only have a h2o since friday)
But if price is more important to you than these features, I understand