About contrib module being free: OpenCV main library is free, under Apache 2 license, and previously under 3-clause BSD, as you can read here.
Contrib module is a separate one because it contains code not compatible with this license. So, you can download it and install it for free, but there may be obligations when you use it, like free to develop but not for commercial use, and other like such.
But also contrib has code “in the process” to be porting to the main library. Not all code in contrib module has license incompatibility.