I don't think it's so much a push, more a clear-out of stock !
Our local W'stones has a sign on its (reduced size) ereader display saying it now only has accessories for 2 makes ( River + another, not Sony interestingly)
And all the boxes etc.. on the generic rack look like they've been around rather too long, and are empty anyway, and have one 350, & no 650's, and, yes, no accessories ............
And the £111 offer publicity has disappeared from ours......
And I don't think Sony signed up W'stones with any conviction, really - more named them as "preferred vendor" or some such...
And the chain never had a lot of liking for the idea, in my opinion !
Either way, they haven't what I would call "pushed" any reader much, 'cept when they only had Sony - behind a locked cabinet that you had to be "supervised" to access .........
Can't help thinking W.H.Smiths would have been a better bet - more "populist" !