DeKalb County Online has the scoop, in the form of an e-mail from the FAA:

Partial closures, which I hear Corn Fest is, still requires approval by FAA. And if Corn Fest has previously been held at DKB and closed part of the airport for a weekend, I would say that it should have been coordinated.

[…]

Of particular importance is the amount of money received by the airport from the event. This MUST be some form of Fair Market Value revenue received from the event and put into the airport operations fund. Failure to do so could put the airport in noncompliance with its Grant Assurances. IDA/FAA may request proof of payment to ensure compliance. I would also add that a token sum of something like $500/day or $1,000/day for what appears to be a pretty big event is not considered Fair Market Value.

How did we end up with an airport manager who does not know that a) closures of the airport must have prior approval and b) the airport is supposed to be a money-making venture.