This is due to the fact, that db4o has a unique API, hence the project that uses db4o is derivative work. This fact obviously is very dangerous for Open Source project that do not intend to use the GPL license.
However since recently there is a third licensing option as can be read in the January Newsletter of db4o:
"db4o has issued a third licensing option for free/open source projects that want to embed db4o but do not want to (or are not able to) license their derivative work under the GPL in its entirety. This initiative aims to proliferate db4o into many more open source projects by providing compatibility for projects licensed under Apache, LGPL, BSD, EPL, and others, as required by our users." (quoted from the newsletter)From my point of view, I am happy about this decision, as it makes a pretty interesting technology more accessible for a broader community of developers.
Read more about Persistence in the Java Environment, also covering db4o in the report following our "Java and Persistence" event.