"Before the current reserved name policy was imposed in 1993, Jon Postel (under the IANA function) took steps to reserve all available single character letters and numbers at the second level for future extensibility of the Internet (see 20 May 1994 email from Jon Postel, https://web.archive.org/web/20100301054658/http://ops.ietf.o...)"

Although I can't find off hand where he said about the corporations but I do vaguely remember reading that at some time as well, may be wrong on that specific point though.