The addition of DHCP and DNS in IPAM 3.0 was to allow for DHCP scope and DNS zone monitoring through IPAM. In order for IPAM to perform a DNS lookup of an IP in a subnet, it has to be able to successfully ping the IP and show it is used. Otherwise, if there is no response, the program will list it as available and not check DNS. I'd imagine if this behavior would change then the licensing of IPAM would have to change to be licensed on each IP that pulls DNS records. This would effectively eat up your licenses quicker. As it stand now, no available IP is counted in the license, so no action is going to be performed on it.
Thanks Matthew, I hadn't really thought about the license implications. I was just hoping it would take the data it extracted from DNS to populate the hostname column of the subnets. It already knows there's a DNS hostname to go in there as the column at the end acknowledges a DNS record. The IP could still show as Available I guess and would not show any other fields populated. Wishful thinking on my part I guess!