large organization nameservers sending icmp packets to dns servers.

[Drew Weaver <drew.weaver () thenap com>]

        Is it a fairly normal practice for large companies such as Yahoo! And Mozilla to send icmp/ping packets to DNS 
servers? If so, why? And a related question would be from a service provider standpoint is there any reason to deny 
ICMP/PING packets to name servers within your organization?

Thanks,
-Drew