I believe McMillan's position is just as you said, LDB .... only one copy in the expectation that people will buy vs borrow. They do allow purchase of multiple copies after a certain period I believe, but I'm not sure if it's just "12 weeks" or whatever, or some period after it's not on the bestseller lists any more. I agree, though, borrowers and buyers are two very nearly mutually exclusive sets of people. At least as regards any given book.
As to the library, while I understand their position, to me it's like what my mother called "biting off your nose to spite your face". So not only can the library only have one copy, they're mad about that so they don't purchase ANY. How is that good for their patrons? I guess they're hoping their patrons will have a go at MacMillan ... but I suspect that most of them haven't a clue about any of the underlying issues. All they know is the book is/is not available.
They may also not be realizing that many of us have legitimate access to more than one library system. I have cards at 4, and could add 3 more if I wanted to go to the trouble. (I recognize this may not be the case in a more rural area.) If a book isn't carried at one of them, it's a pretty good chance it will be carried at one of the others. Either way, I don't mind a longer hold period as there's plenty else to read in the mean time. I can count on the thumbs of one hand the number of authors I buy rather than wait to borrow from the library.

In some cases that may mean I never read the book, or at least not for months or years later. And that's o.k. with me.