Why is the software under the Public Beta Program considered confidential?
One reason beta software is restricted and considered confidential is that by exposing features could result in revenue recognition issues for the company, which could result in the company refunding money or needing to restate their corporate financials.

Lets say you play with the beta, and are extremely excited to discover that this beta has support for left-handed keyboards. As a big user of left-handed keyboards, you begin tweeting about the support, write blog posts, etc.  Many users decide to purchase new Macs because they too are anticipating left-handed keyboards. However, Apple decides that supporting left-handed keyboards doesn't work well enough, and decide to not ship the support in the golden master release. Customers sue Apple, demanding not only refunds but reparations for lost time because of the "missing" support. The SEC investigates Apple, because they feel Apple may have mislead investors, who anticipated that there would be lots of sales of Macs, partly due to left hand keyboard support....

You get the idea.