Apparently us-east-1 is backed by Dyn (and only Dyn) as well?
$ host -t NS us-east-1.amazonaws.com
us-east-1.amazonaws.com name server ns3.p31.dynect.net.
us-east-1.amazonaws.com name server ns1.p31.dynect.net.
us-east-1.amazonaws.com name server ns2.p31.dynect.net.
us-east-1.amazonaws.com name server ns4.p31.dynect.net.
That's… utterly bizarre to me. us-east-2 has a more diverse selection:
$ host -t NS us-east-2.amazonaws.com
us-east-2.amazonaws.com name server u4.amazonaws.com.
us-east-2.amazonaws.com name server u6.amazonaws.com.
us-east-2.amazonaws.com name server u3.amazonaws.com.
us-east-2.amazonaws.com name server u2.amazonaws.com.
us-east-2.amazonaws.com name server u1.amazonaws.com.
us-east-2.amazonaws.com name server u5.amazonaws.com.
us-east-2.amazonaws.com name server ns2.p31.dynect.net.
us-east-2.amazonaws.com name server ns1.p31.dynect.net.
us-east-2.amazonaws.com name server pdns1.ultradns.net.
us-east-2.amazonaws.com name server pdns5.ultradns.info.
us-east-2.amazonaws.com name server ns3.p31.dynect.net.
us-east-2.amazonaws.com name server ns4.p31.dynect.net.
us-east-2.amazonaws.com name server pdns3.ultradns.org.
Not that anyone should be running a service whose availability they care about solely in us-east-1 anyway…
$ host -t NS us-east-1.amazonaws.com
us-east-1.amazonaws.com name server pdns5.ultradns.info.
us-east-1.amazonaws.com name server ns3.p31.dynect.net.
us-east-1.amazonaws.com name server pdns1.ultradns.net.
us-east-1.amazonaws.com name server pdns3.ultradns.org.
us-east-1.amazonaws.com name server ns4.p31.dynect.net.
us-east-1.amazonaws.com name server ns1.p31.dynect.net.
us-east-1.amazonaws.com name server ns2.p31.dynect.net.
us-east-1.amazonaws.com name server u1.amazonaws.com.
us-east-1.amazonaws.com name server u2.amazonaws.com.
us-east-1.amazonaws.com name server u3.amazonaws.com.
us-east-1.amazonaws.com name server u4.amazonaws.com.
us-east-1.amazonaws.com name server u5.amazonaws.com.
us-east-1.amazonaws.com name server u6.amazonaws.com.
us-east-1 is the oldest region and predates Route 53. Not adding extra DNS providers to the older regions is probably an oversight.
(The EC2 API team requests load balancers from a separate load balancer team. The load balancer team probably didn't exist as a separate team when some of these regions were created.)