Technically you are correct it's built off RHEL, but they are not working like a RHEL/CentOS system.  Every single Forge package I've tried to use that has used osfamily = redhat or operatingsystem = Centos does not work (or at least didn't).  Is it possible at some point something changed in the Amazon Linux AMI that broke Facter or Facter had a bug?  Just a few weeks back operatingsystem was = to Linux on Amazon Linux.

[root@puppet ~]# facter operatingsystem                     
Amazon
[root@puppet ~]# facter osfamily
RedHat
[root@puppet ~]# cat /etc/system-release
Amazon Linux AMI release 2013.09

[root@puppet ~]# rpm -qa | egrep "puppet|facter"
puppetlabs-release-6-10.noarch
rubygem-puppet-lint-doc-0.3.2-1.el6.noarch
puppet-server-3.4.2-1.el6.noarch
rubygem-puppet-lint-0.3.2-1.el6.noarch
facter-1.7.4-1.el6.x86_64
puppet-3.4.2-1.el6.noarch
puppet-dashboard-1.2.23-1.el6.noarch