Once jamvm is installed, it is listed in /usr/lib/jvm, so I guess the intent is that it can function as a system VM. However, jamvm rdepends on gnu-classpath, which rdepends on virtual/jre-1.5 (since gnu-classpath also ships binaries that need a JRE). So in the end, JamVM rdepends on a bigger JRE, which should not be the case, I think.
Added to jre:1.5.