Most of the packages I have seen on Hackage are libaries released with open-source licenses and I think I have faint memory of a hackage upload with a missing license field triggering a bashing from hackage.haskell.org about not using an open-source license.
Is there a rule that says every package hosted on Hackage is required to be made available under an open-source license?
If you claim that there is such a rule, how does http://hackage.haskell.org/packages/archive/Cabal/1.14.0/doc/html/Distribution-License.html relate?