Why can't I buy this book?

When my son and I were in Boston last month we stopped by Peacefield, the homestead of American Revolutionary hero and later President John Adams. We just wanted to see it; I'm an admirer of President Adams and we had a few minutes, so Frank looked it up on his smartphone and we found it.

Once there, we walked around the open grounds and took some pictures. You know, touristy stuff.
Outside the carriage house we were stopped by a staffer. There was no problem. He just saw us there and asked what he could do. We explained we were just looking around, and he took us inside to tell us about available tours, which began a couple miles away.

As he talked I saw a small display of books and memorabilia, obviously for sale. There was a biography of Abigail Adams, John's wife. I have read several volumes on the President; I thought then I'd buy one on Abigail, to hear more about her and her perspective. "There's plenty of books and such at the visitor's center," I was told. Okay, thank you, I replied, and we left.

That's what happened, in a nutshell. But I wondered then and wonder now why he wouldn't sell me that book. In my mind that conversation went kind of like this:

"I would like to buy this biography of Abigail Adams please."

"Oh, there are plenty of books at the Visitor Center."

"Okay, but I'm here. I would like to buy this book."

"You'll find all kinds of books and gifts at the Visitor Center."

"I'm sure I will. But I'm here and so is this book. May I buy it?"

"Memorabilia, glassware, collectible plates, pictures..."

"Why won't you sell me this book?"

"It only takes about two hours; you'll really enjoy the tour."

"I'm not arguing that. I just want to buy this stupid book, that's all!"

A hush slams throughout across room. "I find your lack of respect for one of our nation's finest First Ladies appalling," replies the docent. "Good day." And we leave without the book.

It didn't actually go like that, as I said. But it would have been really easy to sell me that book.