This is a VERY SIMPLE toolkit. It is not meant for writing extensive desktop applications (yet). Small simple ones with simple needs.
It is meant to make the programmers work almost brainless but give them lots of flexibility.
One can divide Elementary into three main groups:
LGPL v2 (see COPYING in the base of Elementary's source). This applies to all files in the source tree.
There is a lot that goes into making a widget set, and they don't happen out of nothing. It's like trying to make everyone everywhere happy, regardless of age, gender, race or nationality - and that is really tough. So thanks to people and organizations behind this, as listed in the Authors page.