This week's book giveaway is in the Mac OS forum. We're giving away four copies of a choice of "Take Control of Upgrading to Yosemite" or "Take Control of Automating Your Mac" and have Joe Kissell on-line! See this thread for details.
The Oak 0.2 specification appears to suggest this was a performance optimization.
Although each Oak compilation unit can contain multiple classes or interfaces, at most one class or interface per compilation unit can be public... This restriction is ... necessary for efficient package importation.