There's complying with terms and conditions and there's not accepting the consequences of a mistake made (as you so eloquently stated). However, I would go so far as to call the action fraud (definition: an act of deceiving or misrepresenting) although a legal claim of fraud would be a challenge (as miles don't belong to a customer). That said, the miles could very easily be removed / account suspended. While I don't have the NW T&Cs at the time, most airlines have pretty consistent policies in their mileage programs. Here's a few snippets from the airline where the mileage balance would have been transferred after WorldPerks ceased to exist:

Mileage Credit Rules & Conditions:

Altered or illegible documents will not be accepted. Alteration of documents or other attempts to obtain mileage credit to which the member is not entitled are prohibited.

SkyMiles Rules & Conditions

Delta reserves the right to terminate your membership in the SkyMiles program at any time if you violate the SkyMiles program rules, any term or condition of Delta's contract of carriage, Delta's fare rules, or any other Delta rules and regulations that apply to your travel. Termination of your membership will result in a loss of all accumulated mileage credit in your account, the cancellation of any unused Awards or Award Certificates, and the loss of all other SkyMiles benefits. Terminated members are not eligible to participate in any aspect of the SkyMiles program, including without limitation any special promotions or SkyMiles partnership offers. In lieu of termination, Delta may at its sole discretion deduct mileage from your account, but permit you to continue participating in the SkyMiles program.