I think Ray's correct in that SME doesn't use many of the modules that standard RedHat will. In trimming down the number of installed modules, it helps keep the system more secure; fewer installed modules/running processes means less vulnerabilities. This also means when RedHat posts security issues, these issues may not apply to SME because the module may not be there. Take a look at the install for RH and compare that with SME. Last time I downloaded RH ISO's, there was 3 install CD's and a separate documentation CD where SME is a trim single CD install.
Another thing is that SME only firewalls the public NIC in a server and gateway configuration. In server only, there is no network protection (firewalling) enabled. This is by design and I believe it's a good one. The belief is if it ('it' being any TCP/IP packet/datagram) is on the internal network, it's supposed to be.