I don't think it's confusing at all. The comments about how long your credits last are based on ACTIVE membership. If you choose to quit, you are no longer an Active member. They give you a STRONG warning that you will lose that credit if you quit. So use your danged credit.
Also $15 for an audiobook is actually very reasonable. Most retail audiobooks are $25-35 and more. Absolutely true that many of us seek the lowest cost, but $15 for next in the series of a series you really enjoy (and will likely listen to again) is not all that much. A 2.5 hour movie on a Saturday night is $13.95 in my area. Sales are promotions of authors, just like at a regular bookstore.
Yep, I agree PWalker, if most of your books are UNDER the cost of a membership, then it's harder to use your credits. I have no shortage of books that cost more than a credit, so not a problem for me, but we read different things.