>It is usually in the interest of the designer of the API to have as many users as possible.
When they first design and publish it yes, but Oracle didn't design and publish the Java APIs, they bought them after they had becomes very popular and widely used.
At that point they don't need to care about making it popular, it already is, all they care about is milking that existing ecosystem for every penny they can get. If future API publishers care about this they can just license out their API spec, but right now Oracle just wants to be able to squeeze whoever they can until the pips squeak.