This could be related to the fact that soundcloud is an european company and none of the legislation in Europe actually provides a means for donating works to the public domain (see http://en.wikipedia.org/wiki/Public_domain#Dedicating_works_...)

I remember the creator of SQLite talking about this on a very old episode of the FLOSSWeekly podcast where he said that this was somewhat hindering adoption because in Europe the legal status of SQLite wasn't clear.

He solved that by allowing us to buy an SQLite license for 1000 dollars, though :)

I wonder how many of those have been sold!