Under a 1956 consent decree in settlement of an antitrust case, AT&T (the parent organization of Bell Labs) had been forbidden from entering the computer business.. Unix could not, therefore, be turned into a product. Indeed, under the terms of the decree, Bell Labs was required to license its non-telephone technology to anyone who asked.

AT&T made Unix available to universities and commercial firms, as well as the United States government, under licenses. The licenses included all source code including the machine-dependent parts of the kernel

-- http://en.wikipedia.org/wiki/History_of_Unix

So, while they did not actually release it to public domain, they got about as close as they could have without actually doing so.

Remember the GPL didn't exist until 1989. Also, I would speculate the generally free access to Unix V probably helped bring about the modern environment of open source, which didn't really exist then.