All rechargable batteries wear out, all of them. Even the long lived wet-cell battery in your automobile has a lifespan.
Some manufacturers will sell product at low margins designed with proprietary batteries only THEY can supply. The accessories, such as batteries, are then REALLY jacked up in price. Also, older generation batteries can be shipped with the product, not the latest technology.
Not saying MacApple is doing this.....but it would not surprise me.
The work-around for this problem is after-market battery suppliers IF they are allowed to manufacture iPod replacements. One way to watch MacApple on this is to see if they go after after-market battery suppliers in court.
If not, google "replacement batteries ipod" and check out the milliamp hour (mA H) ratings. Generally, the larger, the better. Also, Lithium Ion types usually are not known for memory problems.
Maybe some other techno-FReepers can dig into this further. Battery technology changes FAST.