The OSL 3.0 license does not have any problem with this, however, the FSF says that the GPL does. Incidentally, the FSF would undoubtedly make the same statement in regard to the license for CodeIgniter 1.x and 2.x, so this is not a change.
OSL 3.0 has a more defined view of “derivative” and only considers actual modifications to code - not merely linking other source code - as creating a derivative work. On the other hand, the FSF interprets the GPL in a very strict way, that any distribution that includes GPL code must in whole be licensed as GPL. The reach of the license of GPL’d code is hotly debated among lawyers, which is itself a significant reason to avoid it.
Since CodeIgniter is a platform, we think this restriction is icky, and applications you write - your own code - should not have our (or anyone else’s) license forced upon them.
If you were used to the old proprietary CodeIgniter license, the most notable changes are: