The reason is that a lot of people use Lenny (and or Ubuntu, which would cause this same issue), and Squeeze/Sid aren't stable yet (even though they work fine, I know that). Every indication I've gotten is that "we" prefer packaged versions of things instead of pulling direct from CPAN. I chose to eliminate the error message when installing on Lenny/Ubuntu from Makefile.PL, and allow a required version that was lower than the current version to help eliminate a bad user experience (An error!!! OMG! What do I do!).
I truly don't care which way it's done: we can remove it from the package script and add it to the modules requiring installation through CPAN, no problem. I'll do those patches too, if necessary.
That said, I don't think the reasoning behind this particular patch is unsound, based on past precedent.