The difference, at least from my point of view, is that it is a realistic deal to find a pair of shoes for $39 dollars when I don't think it's reasonable to be able to buy X cds for a penny, for even X=1. The closer you get to the actual or expected value of what you're buying, the more unethical I feel it is, just because you're going to be tricking a greater number of people.
If you bought into Columbia House in the 90s, as I did, the math worked out to ~$10 average for each CD over a year (the 8-for-a-penny plus the full-price ones), back when they were $16 each at the local store.
The promotional offer was an attention-catcher, but you still saved $4-6 per CD.
Just because you feel like "there must be a trick" doesn't mean there is a trick.
The cleverness of Columbia's approach was that a certain percentage of people at the margin would keep (and pay full price for) CDs that they'd never have bothered to go out any buy on their own. Just like credit cards: Issuers hate people who pay the balance off every month and earn them zero interest -- but most consumers aren't quite that disciplined.
