The bigger difference, to me, isn't so much that the walled-garden ecosystems restrict a phone/tablet's use as a general-purpose computer, than the form factors and interfaces of the device being appropriate for different forms of content creation, manipulation and consumption.
Phones and tablets are great for consuming certain types of content, and having a portable camera with networking has enabled a lot of content creation. But a phone isn't a great device for doing something like long-form writing.
I don't know about you, but for most reviews and new articles it easier for me to put it on in the back-ground than having to deal with advertisement and pop-up windows on web-pages when reading text.
Phones and tablets are great for consuming certain types of content, and having a portable camera with networking has enabled a lot of content creation. But a phone isn't a great device for doing something like long-form writing.