The original Techcrunch article didn't have "(YC 07)" in the headline of their post. Add it while posting on Hacker News, and you get upvotes immediately and may be blindly! Good trick!
This is standard procedure. Remember how this site started: it was originally for YC "alumni" to exchange news. Most visit the site regularly, and they are genuinely interested in news about their peers. Considering there are on the order of 300 founders now, it's surprising articles about YC-funded cos don't get more upvotes.
Agree it's standard procedure (and that it is a useful hint to all readers), but I thought the original purpose of the site was to engage potential YC applicants so you could better evaluate them.
That was one of the side benefits, but the main thing was just that we wanted a good news site.