I can certainly understand why the organization and partnering businesses would require a registration and then a call ahead for a "reservation." Just looking at the map on their web site for Indiana alone, I'm familiar with the locations of some of these sites. Yes, it's kind of a hidden secret, but this prevents just anyone from pulling into an open field and spending the night, and then hitting the road in the morning.
Consider if it was your property? You wouldn't just want anyone dropping in, squatting for the night, and then moving on. By requiring "membership" the owners of these companies and businesses are dealing truly interested visitors who want to experience part of the inner-workings of how the business or company operates.
Purchasing from them is a great courtesy and does help support their business. We visited a honey farm in Southern Indiana a few years ago and took our granddaughter. They provided overnighting for RV's, but it was basically just a couple spots in their graveled parking lot. Still, it was a nice offer. We took the entire tour, my granddaughter got to make real bees wax candles, and I think we walked out with about 3 jars on different flavors of honey. We did not spend the night. The folks were extremely friendly, knowledgeable, and it was really a very fun day. Purchasing something from them was a real joy, not a pain in the pocketbook at all.
Years ago, I tried to persuade my in-laws, who were heavily into farming with the multi million dollar tractors and combines, and farmed about a thousand acres of field, to start a business called, "Farmer Dude". They had the property to set up an area for campers to park. We parked ours there during the harvest season and would help out. I drove the trucks from the field to the elevator. The life-style was great! Hard work, yes, but so very much fun at the same time. I tried to persuade them to do "Farmer Dude" ... set it up like a "Dude Ranch", except having a bunkhouse, folks would stay in their own RV's. Unfortunately, they never bought into my idea.
I think Harvest Host is a great idea. The actual organization of "Harvest Host" happened long after I came up with the idea of "Farmer Dude."
My father-in-law is now gone, my mother-in-law has sold off all the property except 4 acres around the house. His brother has continued the farming, but he's down sized quite a bit now. An opportunity lost forever.