"I wonder how much of the technicalities are enforced by registrars "
In the absence of any incentive to do so? None.
You see, ICANN enters into all sorts of contracts with registies and registrars to do particular things. But these contracts never meet the basic test of contract drafting - "If party A is supposed to do X, but doesn't, then what happens?"
These things are written as all-or-nothing propositions. Is Neulevel going to have its status as .us registry revoked because some domain name doesn't comply with the nexus requirement? Are you kidding?
From the perspective of a registrar or registry, a non-compliant registration brings in the same number of dollars as a compliant registration. If a particular legal problem arises in connection with a given domain name, then, sure, they have the discretion to "enforce" a rule. However, they don't care to whom a revenue-generating name is registered.
See
http://www.circleid.com/articles/2537.asp