I think they are talking about iOS and Windows Phone 7/8 and Windows 8 RT that is being pushed on tablets and phones. There is no way to side load your own apps and anything that needs to be distributed is through a curated (and at times censored) store that is controlled by MS and Apple. The stores are curated not just from a content perspective but also based on what could duplicate or compete with their own offerings. (Something for which Microsoft got smacked down hard in the EU already). They are getting away now by neither having a monopolistic market share individually.
Look at the iOS limitation of something as basic as not being able to set default apps.
Look at WP7's limitation of not being able to change your default search provider. (Though I do believe this has changed in WP8).
Look at the iTunes store's contracts with publishers and developers where they are not allowed to sell the same content at a cheaper price elsewhere.
Most people aren't bothered by this at this point (including me - I jumped from iOS to Android, but not for these reasons) but the FSF cares and can see where things are heading. There is now a Windows 8 and OSX app store. What happens if Apple offers Laptops and PCs with a $200 discount that can only install software from the App store. Or MS a free Version of Windows 9 that can only install apps from their windows store. (Remember, they get a 30% cut of everything sold through there). There might be a large percentage of people open to that.
I do agree this is a slippery slope argument, but look at what the state of mobile OSes would have been if Google hadn't entered into the picture. (And this in about 5 years or so).
