The ampersand is at the beginning of HTML entities (remember, we talked about them back here). And then you run into programmers thinking like programmers, not like people.
You see, there is a html entity for the ampersand: & (Yes, I had to render that with HTML entities, and it hurt my brain. See what I do for you?) Programmers who have programs parsing HTML and XML (remember, your eBook is made up of this stuff) expect you to use that entity when you want to use that symbol. Even if you don’t use it directly in the eBook itself, perhaps you’re uploading an XML document somewhere – say, to get a book in Kobo.
And if any of that HTML or XML has a straight ampersand in it? Then everything gets darned to heck.