Self-pubs and publishers enrolled through KDP do have control over enrollment, but Harry's question was about trad pub books available in KU. Larger publishers typically don't go through KDP, they have separate contractual arrangements with Amazon.
Here's a quote from Publisher's Lunch (16 July 2014) when KU was first announced:
"...Amazon already had a big, direct license for Harry Potter for Kindle Owner’s Lending Library. The company had previously offered Hunger Games for borrowing through the Kindle Owners Lending Library, though they did so without any specific permission from Scholastic, which was paid full price on each lend. Scholastic spokesperson Kyle Good confirms to us that it is “the same situation” for Kindle Unlimited — Amazon informed Scholastic they would be including the books in this program and “they have the right to do it” under their current contract, though there was “no new negotiated deal” specifically covering this usage. As with KOLL, Scholastic will get paid their full wholesale price every time one of their ebooks is opened by a Kindle Unlimited subscriber.
Other wholesale-basis publishers who declined Amazon’s offers to authorize participation have reported the same thing as Scholastic: That Amazon told publishers the etailer has the right to include ebooks in the initiative without permission, as long as they pay for each open as a regular sale. Offers made to publishers to directly authorize participation were similar to the way existing subscription programs such as Oyster and Scribd operate, where the publisher is paid full price after a certain specific percentage of any book is read."
More recently (few months ago), Kensington CEO Steven Zacharius commented on a publishing blog that Amazon was removing Kensington titles from KU at their choosing. So I would say, for some trad pubs Amazon can and does exercise a right of KU enrollment (or removal.)
The largest number of books from any one trad publisher I've seen in KU (4,000+) is Open Road Media. I would guess they have separately negotiated contract terms, but none of us really knows for sure about any of this, it's all inferences, some more educated than others, based on media reports.
http://lunch.publishersmarketplace.c...ffer-probably/