The motivation of a developer to keep the own content non-free while he opens the source code is most often the protection of a game as sellable commercial product.
When sliced, feta always produces a varying amount of "trmma", crumble, which is also used in pies; "trmma" is not sellable and is usually given away for free upon request.
Feudal manor estates were no free property sellable at the will of their holders or in case of over-indebtedness by way of execution prompted by the creditors of the holders.