There are certain circumstances where private network operators may wish to use their own domain naming scheme that is not intended to be used or accessible by the global domain name system (DNS), such as within closed corporate or home networks.
The "internal" top-level domain is reserved to provide this purpose in the DNS. Such domains will not resolve in the global DNS, but can be configured within closed networks as the network operator sees fit.
This reservation is intended for a similar purpose that private-use IP address ranges that are set aside (e.g. [RFC1918]).
The US Government ended its involvement in IANA when it moved to an international oversight model in 2016. IANA is operated by Public Technical Identifiers which is an affiliate of ICANN, and ICANN provides all of its funding. Governments have a say in how ICANN operates through its Government Advisory Committee, at which all governments — including the US — have a seat.
It says it is licensed under Apache License 2.0, but also under "Prosperity Public License 3.0.0" which limits use for a commercial purpose to 30 days.
Are you sure its not in reference to "Greek" the language, rather than "Greece" the country? "el" is coding for Greek in ISO 639, IANA language subtags, etc.
The history on why some external territories are coded is because an international standard is the basis for the two-letters used for these TLDs, and that standard is used for other purposes. Far flung localities are often coded due to things like physical mail delivery or customs boundaries which utilize the same standard.
Why does .eu exist? Essentially because "eu" was coded for the European Union to facilitate "EUR" as a currency code when the Euro was introduced. ISO 4217 currency codes derive from ISO 3166 country codes.
ICANN is finalizing a policy that will govern this question. In short, if adopted, 5-10 years after the country is removed from the ISO 3166-1 standard it will be removed as a TLD.