The operators knew. They just didn't care. Or they may not have known that the tower had been stolen and assumed the transmitter was broken and were procrastinating to fix it.
Moreover, I can't imagine anyone successfully tearing down a working antenna. My guess is the timeline went: transmitter stops working, site sits idle for a few months, thieves notice it's turned off, raid the place, station owner finally decides to do something about it, discovers they've been cleared out.
Taking down a tower requires more time and care. And tools. And it pays less for scrap, although the black market for the kind of equipment you find in a transmitter building is small and not well-capitalized.
Telemetry is cheap in the scheme of broadcast expenses.
If the station is operating on a shoestring budget, you have at least one human who cares that the revenue-generating operations (broadcast signal) are happening.
Similarly that person would notice the change in power expenses.
Even in the lowest-tech, least-compliant of stations, someone would notice an antenna and/or tower loss within a day. This reads like a case of severe neglect, almost certainly intentional, and possibly fraudulent.
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.
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.
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.
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.