To add to Martin's answer, I choose to not charge for mine (typically webparts) because I feel what I've done will be helpful to others. Now with that being said, if I come up with some hard core module that I've invested a lot of time on and find it would be worthy to market and sell, I'll probably do so. I think it also comes down to a bit of greed. Kentico is different in culture when it comes to a CMS and that's why I chose to partner with them. That point was made clear when I went to Denver to the Connections conference in 2012, how many other software development companies can you sit 3 feet across a table from and review source code. Or provide your input for future versions in focus groups and see those ideas turn into reality in a short amount of time?
Kentico offers a lot out of the box and many webparts you'll find are simply minor modifications to other webparts or controls Kentico has already developed. I'd say take advantage of the free items, be sure to look for the "Kentico Tested" and test and review any code prior to putting into a production environment.