Anything that pertains to Portage
Using a minus sign in front of USE flags to switch things off is normal practice, and allows total flexibility, but I think your question is "Why would one want the ebuild to delete the kernel and initramfs in /boot when downloading just the sources?" Perhaps lxnay had a reason for doing it that way, I'm not sure. Perhaps to wipe the slate clean before starting to build a new version of the kernel? You could always file a bug report in the SL Bugzilla; it wouldn't do any harm. That said, the new version of the ebuild, for the 2.6.30 kernel, has been split into two anyway as I mentioned in a previous post, so presumably the point becomes academic.