I agree it is a lousy situation.
I also have a difficult time understanding the cheater sized tank.
The following is not a justification, just noting what seems to be an unwritten rule or code.
Once a series becomes a little more than Saturday night mains, very few will get a checkered flag taken away (again note the exception of the local weekly races, were protest and a winners resulting DQ isn't unusual).
Promoters and sanctioning bodies want fans going home certain of who won. They may fine enough to make it costly, even take points, but they are very hesitant to scratch out the winners name.
Not saying it is fair, just the way it usually works.
The only two exceptions I recall: Nascar penalizing Rudd, and awarding Davey Allison the win (Rudd knocked him out of the way to pass him late in the last lap at Sonoma). Logano getting DQ'd at Irwindale for a similar move (less sure of the Lagano episode, I think that was the case).
A different series, a different scenario or infraction, but I think the principal works the same.