Used-to be linux-ware. Actually lots of its initial features got moved to the more linux-centric cups-filters.
Both upstreams are really easy to work with and accept portability patches with pleasure...
But I disgress, the original question was regarding the Tux logo. The reason is simple, it's just a default print test page; actually most linux distros ship with their own modified^branded one.
Unless someone comes up with a super cool puffy print test page, I don't see the point in changing the default; who cares whether you are printing a penguin or a monkey eating bananas... it's just a test job.