Lee is right. :) The OSS xtUML Editor is licensed under one of the most liberal licenses available (Apache). There are virtually no restrictions on its public/private/commercial/educational use.
The proliferation of Executable UML is a Good Thing for the business and the engineering community. More modelers mean more people who want to execute and translate models. Some people may be willing to pay for commercial grade execution and translation. Those who do not can build their own. The APIs are opened up.
In any case, the/an editor is the critical front end to the process. It needs to be widely available and accessible. Thus, it is now free.
I hope that more people build execution engines and translators. It is the Right Way.
xtUML Modeler, Teacher and
Mentor Graphics Engineering Manager