I wonder how much this has to do with BSD3 being the default license when you use the "cabal init" command to generate a project skeleton. Most people probably don't care which open source license they're using; of those who do care, it's generally GPL versus everything else.
I don't think it has much to do with "cabal init". I've used Haskell and noticed that most libraries are BSD3, so I make my libraries BSD3 also. I did not know that there was such a thing as "cabal init" until just now :)
Over in the Perl world, most libraries are GPL2/Artistic, so I follow that convention. In the end, I would probably prefer the MIT license for libraries... but following convention seems better than setting my own.
(For applications, I go with the GPL3 because it's not going to inconvenience anyone and it provides the best protection of free software. But people are just going to skip your library if they have to relicense their project, so I find that the GPL is not particularly appropriate in that case.)
For me it's a conscious choice. On general principle, I think that open-sourced library code at least should almost always be BSD-licensed. Furthermore, there are many good libraries on Hackage which I might well want to use in a commercial product some day, and a BSD license will make that easy, no matter where my code ends up running. Releasing my own code under the BSD license is therefore also a small gesture towards maintaining this state of affairs.
