Yes, if you choose a "Custom" install, you can control which browser will be the default. But most users will choose the express option; and if they've already selected a default browser other than IE, why not leave it that way? This isn't accidental behavior; it was intentionally done by Microsoft, and today things like these don't go unnoticed.
Microsoft, of course, defends by saying that Windows 7 RC is just a test release, not intended for wide use. But it's a new version of Windows; it's interesting to many, open to everyone and free for a year. My guess is that a lot of users will be experimenting with it, and if only a fraction of users switch back from an alternative browser back to IE, this will translate into a web browser market share percentage increase for Microsoft. Or at least a free, very effective ad for IE. As Mozilla’s chairwoman Mitchell Baker puts it, "Our initial review suggests this is a blatant use of the Windows operating system to change the market dynamics of browser usage."