It is just anti-competitive censorship. In the past, Apple has done things like banning an Android-focused magazine app [1]. There have also been controversies over books linking to Amazon when referring to other books you might buy (as their print editions do) [2]. This Treehouse situation fits the same pattern.

[1] http://tech.fortune.cnn.com/2010/11/26/apple-bans-android-ma...

[2] http://paidcontent.org/2012/02/29/419-who-decides-what-gets-...