TBP 3 keeps asking to make itself the default email client.

Just installed the TB3 beta on my flash drive in the default location (\PortableApps\ThunderbirdPortableTest) and it keeps asking to make itself the default client on startup, regardless of whether that option is on or off in the preferences.

Operating system is Windows 7 Ultimate final (7600), TB version is 3.0b4, although I had this issue with 3.0b3 as well. Drive is a 16GB JumpDrive FireFly; while I don't think that matters (all mass storage devices are the same to the operating system), might as well be thorough, right?

I finally got the prompt to go away by unchecking all the options (except for Feeds, which it won't let me deselect--why is it forcing that association, that's not very nice behavior) and clicking OK. I'm wondering if it has anything to do with the fact that Win7 doesn't include any email client and therefore Thunderbird is prompting automatically because it doesn't think it's stepping on any toes, so to speak ("Well, this machine doesn't have an email client at all, so might as well take over.").