An obligation to disclose APIs (and source code after service-shutdown) would also make sense for other applications than games.
I would even go so far as to say that APIs to end users must always be disclosed. Control can still take place via access rights.
Imagine if appliance power plugs depended on the power provider: Coffee machines would come in 'umpteen different plugs, depending on the electricity provider. Crazy! But normal for IT...
#StrangeNewWorld