Well, even if the maintainers have the copyrights, that only means future versions can be closed source. They can't terminate the already-outstanding licenses without a breach of terms.
Oracle doesn't need to take MySQL closed-source or even make any money from it directly. By buying Sun and hence MySQL, they'll disrupt MySQL's development, giving Oracle DBMS a jump in development time, and presumably in features/quality, market share, etc.
It takes time to set up a real fork, and Mr. Widenius may not have the resources to hire as many employees as MySQL AB did without the commercial dual licensing arrangement. As for the development within the MySQL subsidiary, Oracle can slow that down simply by rewriting the release schedule.
In sum, if the goal of this acquisition was to address the threat of MySQL to Oracle DBMS (which I think it may have been, in part), then that task is accomplished not by making money from MySQL but by slowing down its development. Even a difference of 6 or 12 months would be a huge advantage for Oracle DBMS.
That's why I felt that antitrust laws should be considered here, because it's an anticompetitive strategy, but it would be hard to prove because the effect is only indirect.
Also, a less-informed regulator or judge would look at industry figures that show the database market to be highly competitive and not at risk of a monopoly. But industry figures are based on revenue, not adoption, which masks MySQL's true penetration because most MySQL installations are of the free version.