From the NY Post:

The term “walled gardens” refers to a service or technology that restricts access only to those using that service, as is the case with Apple’s iTunes, in which music can only be played on Apple devices or through iTunes itself.

What the hell are they talking about? Apple has been selling music as DRM-free MP3’s since April 2009. I can play my tracks on any computer I want.
A more accurate slam would have been Apple’s DRM-protected video content.