I'm not surprised. Those rivets are part of the barrel plate, which is made of ABS plastic. ABS plastic is known to break down when in contact with petroleum based oils.The penetrating nature of gun oil means it will eventually seep underneath the rivet and reach the other side of the metal base plate.
To stop the oil from attacking the ABS any further will require a full disassembly, a thorough cleaning of everything with a detergent, and of course a screw/bolt mod to put it back together.
Although the metal base plate looks exposed, there is actually a very thin layer of oxide coating it from the sheet-metal manufacturing process, which acts as a rust inhibitor. The oil won't hurt it, but any sanding will also remove this thin layer. While you have it completely disassembled, you may want to consider painting the metal base plate to restore it's protection, and stop any rust problems for good.
IBM intended these keyboards to be used in industrial environments, after all. So long as you aren't storing your keyboards in a dank leaky basement or an outhouse, they should be able to withstand a typical indoor environment. Do you have rust problems with other metal items kept indoors in your house? What part of the world do you live in?