Did a quick google and the first review that came up shows that’s not true at all, it’s the exact same process on a kindle as it is a Kobo, though you and this review are both really over-selling it:

Getting ebooks from other stores onto the device is also a hassle. You have to plug the e-reader into your computer and drag and drop files (though Calibre, the ebook management app, does make it a scootch easier). But that problem isn’t unique to Kobo. Amazon and Barnes & Noble also insist you sideload books.

The real reason seems to be that the Kobo is cheaper, honestly, don’t see why the kindle is that much more