If you compile a binary package, then portage should record the USE flags used when compiling that package into the package binary. This would help building automated installations in a heterogeneous environment. The typical case is that you have packages A and B. With default USE flags, A would depend on B, but you don't want A to use B even though you're going to install B on some computers. So you build a binary package with:
USE="-B" emerge -b A
Now you can't run emerge -K A on a computer where you don't need B at all (even though the binary package A does nothing with B), without having B installed, or you need to supply the USE flag again. The whole process is very frustrating, because you need to record every special USE flag you've used to compile every package...
Steps to Reproduce:
Err... It *is* recorded in the tbz2.
What you're after is the ability to depend (or rdepend in your case) on packages w/ a use flag enabled, which is a dupe.
*** This bug has been marked as a duplicate of 2272 ***