A company's employees publishing open-source projects is, in general, good publicity. Choosing to be not associated would consist of them telling said employee not to use their @google.com account for publishing this project (unless there's some other active publication of 'this is by a Google employee' that I don't know about), and why would they bother doing that unless they thought this particular project would be bad publicity?
(Disclosure: I have been (and will be) a Google intern, which probably affects my priors here, but I do not believe this argument is dependent on non-public information.)
It's not that Google allowed someone to potentially see the connection to Google, but that Google ensured that they (Google) would be associated with the project by informing the media. The story doesn't say "omg through investigative reporting we found that Google paid for the creation of a bitcoin client" (remember, the 20% time is not "whatever you want", but projects approved by management).
Though not yet an "official" project, Google has released a Bitcoin client. As you may remember, there were concerns here about what the government/legal reaction to Bitcoin [1] will be, and the significance of certain groups lending their support to it. EFF and SIAI accept Bitcoin donations, which helps, and this action by Google is another big step.
Previous articles: SIAI accepting Bitcoin donations, Discussion on making money with Bitcoin (Clippy warning on the latter)
[1] In short, it's an anonymous P2P crypto-currency with no transaction fees, in which new units are generated by spending computer cycles computing hashes until you find one with specific properties.