apt-get source ignores default release when source package selected

When downloading source packages by source package name, apt-get ignores the default (or specified) release. This does not happen when downloading source packages by binary package name.

I noticed this bug on our build server, which runs Ubuntu 12.04 but has sources for both precise and raring.

Steps to Reproduce:
1. Add an extra release to sources (i.e. add raring sources to precise)
2. Attempt to download source package by source name:
        apt-get source -t precise twisted
  or: apt-get source -o APT::Default-Release=precise twisted
  or: apt-get source twisted/precise
3. Source for twisted from raring is downloaded. (This is the bad behavior.)
4. Attempt to download source package by binary name:
        apt-get source -t precise python-twisted
  or: apt-get source -o APT::Default-Release=precise python-twisted
  or: apt-get source python-twisted/precise
5. Apt will automatically select the source package from the binary name.
6. Source for twisted from precise is downloaded.

ProblemType: BugDistroRelease: Ubuntu 12.04
Package: apt 0.8.16~exp12ubuntu10.5
ProcVersionSignature: Ubuntu 3.2.0-32.51-generic 3.2.30
Uname: Linux 3.2.0-32-generic x86_64
ApportVersion: 2.0.1-0ubuntu14
Architecture: amd64
Date: Tue May 14 10:42:57 2013
InstallationMedia: Ubuntu-Server 12.04 LTS "Precise Pangolin" - Release amd64 (20120424.1)
ProcEnviron:
 TERM=screen
 PATH=(custom, user)
 LANG=en_US.UTF-8
 SHELL=/bin/bashSourcePackage: apt
UpgradeStatus: No upgrade log present (probably fresh install)