Dual-licensed software will always have grey areas, just by their very nature of being dual-licensed.
As far as MySQL is concerned, the decision whether to pay license fees or not could be helped by answering the question 'Is MySQL essential to the core business?'.
For the asirline online booking system the answer is obviously 'no', whereas for the MMOG the answer is 'yes': the airline is in the business of moving cargo and people around, MySQL is important only insofar as it makes it easier to keep track of everything. The MMOG on the other hand is tightly coupled with MySQL: without the DB, there would be no game.