Learning that this website exists, and then finding detailed and accurate information in my record is what convinced me to start sharing my salary data with my peers. As I see it, this database is a breach of an implicit social contract between me and the companies that I work for. Since my employers apparently have no issues sharing my salary with their peers, I see no reason not to do the same with my peers.
If all employees share their salary information with each other, it becomes a headache for the employer. Each employee may quote the other person's salary demanding a raise. This is the reason behind the "keep your data confidential" idea.
It's also illegal under the NLRA of 1935. Employees talking about working conditions, like compensation, is a legally protected act whether it's on employer time or not.