Just to share my experience following this rule.... I'm in the suburbs of Houston within 5 miles of two very small grass runway airstrips. The rules about letting these small airports know you are flying seem kindof silly, but I thought I'd go through the process. I called the first, and it rang and rang and eventually went to a fax machine. I called the 2nd, (a grass runway airport with "International" in the name, go figure...) and a guy answered. I explained I wasn't flying today but wanted to check the phone#s and the process. He said he gets lots of calls and when people ask if they can fly, he says no. I explained that it doesn't work that way, it's just a notification from the drone owner and he seemed to understand that. I pressed further and asked when I tell him I'm flying, what does he do with that information. Does he get on the radio and tell all small planes I'm flying and what area I'm in? No, he does nothing with the information and eventually it became clear he thinks this rule is silly as well and he doesn't really want my call. The final question was, if I stay under 400' is there really any concern and he replied no, no concern at all, unless I was right in the approach path close to the airport, which I am not.