What was the motivation for revoking the licence to the pilots of Northwest flight 188?
According to this CNN article the FAA revoked their licences with a letter containing the following reasons (emphasis mine):

The letter said the pilots were &quot;extremely reckless.&quot;
&quot;Not only did you not comply with ... [air traffic controller] instructions, you did not even monitor the aircraft's air-ground radios,&quot; the letter said. &quot;You were disengaged and impervious to the serious threat to your own safety, as well as the safety of people for whom you are responsible. This is a total dereliction and disregard for your duties.&quot;
[...]
The FAA said the revocations cite several violations of regulations, including failure to comply with air traffic control and clearances and operating carelessly and recklessly.


As Adam pointed out in the comments, this would violate FAR 91.13:

§ 91.13 Careless or reckless operation.
(a) Aircraft operations for the purpose of air navigation. No person may operate an aircraft in a careless or reckless manner so as to endanger the life or property of another.
(b) Aircraft operations other than for the purpose of air navigation. No person may operate an aircraft, other than for the purpose of air navigation, on any part of the surface of an airport used by aircraft for air commerce (including areas used by those aircraft for receiving or discharging persons or cargo), in a careless or reckless manner so as to endanger the life or property of another.