Given that their license is for a FM rebrodcaster, it seems like the FM business is the one they care about more, and the AM signal was just being used as a means to an end.
My assertion though, is that there's no chance that they were unaware that the AM station was down.
The only path to plausible deniability requires extreme non-compliance, which risks fines and could even threaten their ability to hold other licenses / operate other stations.
So they're in trouble either way, but this story of surprise is absolute crap.
I.e. purposefully don't have any monitoring, purposefully don't go to the site often etc. then only respond of someone tells you there's a problem.
I've dealt with a few contracts in the past that have stipulated that we must respond within xx hours of becoming aware of an issue, if I didn't want to run the service it would be in my interests to do everything in my power not to avoid becoming aware of any issues.
There are regulatory causes for 'xx' response times as well, outside of a contract.
The first part you brought up was willful ignorance[0] which would likely come into play with licensing, insurability, as well as tort and criminal liabilities.
In this case, they were required to operate the AM station to keep their FM license. In addition to the usual threats to license and fines etc, they were also risking their FM license which persumably was ROI positive.
I can see scrappers stealing the tower and nobody bothered to find out. I can also see scrappers stealing the tower, the station knew but since it's negative-ROI they pretended they didn't.