Well at least for MongoDB the reason is quite clear. From their licensing page (http://www.mongodb.org/display/DOCS/Licensing):

   Note however that it is never required that applications using mongo be published.
   The copyleft applies only to the mongod and mongos database programs. This is why
   Mongo DB drivers are all licensed under an Apache license. Your application, even
   though it talks to the database, is a separate program and “work”.