For many the PHP JSON extension license might look like a storm in a teacup, but for many Linux distributions the bits of the free software licenses are very important. That was the case when Debian decided (#692613) to remove the non-free JSON extension as part of the transition to PHP 5.5 in May 2013 (after the Debian 7 release).
This change was done with the help of Remi Collet (from Fedora / Red Hat) who wrote an alternative extension based on JSON-C implementation. A similar change was done by other Linux distributions, and this became the defacto standard for most Linux users.
The situation has recently changed with the acceptance of the Jakub Zelenka’s jsond RFC to replace the current non-free implementation with a free one. The change was committed to the code base on early February (Closing #63520) and expected to be released later this year as part of PHP7.