This is no different to for example the uk, there is no .uk domain.
always. co.uk for companies, org.uk for organisations, gov.uk for governments
ac.uk for educational facilities.
I like this situation as it gives a little insight into the nature of the site first.
This is not a however as stated a second level domain (this is the DR1 bit of the DR1.com DNS), you have your teminology confused. The .com .org bit is a top level domain and the .uk .jp .do is a ccTLD (contry code top level domain.) ccTLDs operate with gTLD according to local policies that are adapted to best meet the economic, cultural, linguistic, and legal circumstances of the country or territory involved. Although it is now possible to register a .us ccTLD it is not often used as most US individuals and companies prefer to have a generic top level domain (gTLD) such as .com .org .net. The US authorities have no problem with using .us on its own, because they do not, maybe as you, see the need for conjoining TLD's
The www. piece is the third level domain.
Hope this provides a little insight.
Jon