currently, the jbpm installation into jboss is mandatory. you have to point the installer to an empty directory if you don't want to install it in jboss.
installation of jbpm into jboss should be optional.

When fixing this, also make sure that the docs are updated: currently there is a section like this in chapter "Getting Started":

"
If you don't want to install jBPM in JBoss, then you'll have to point the installer to an empty temporary directory where it can put those files anyway. This limitation will be removed in the next version.
"