use TemplateDirectiveModel instead of TemplateTransformModel in org.apache.struts2.views.freemarker.tags.TagModel.

http://freemarker.sourceforge.net/docs/api/freemarker/template/TemplateTransformModel.html

Objects that implement this interface can be used as user-defined directives (much like macros); you should rather use the newer TemplateDirectiveModel instead. Altough implementing output filters is more handy with this interface, this interface will be certainly deprecated as superfluous, starting from FreeMarker 2.4. But as far as the template engine is concerned, you can use both equivalently as a user-defined directive.