
The reason for this ban is because BMW's German website used blackhat tricks to manipulate search engine results.
BMW admitted using so-called "doorway pages" to boost search rankings, but denied any attempt to mislead users.More details at BBC.
BMW's activities were revealed in a blog by Google software engineer Matt Cutts. BMW's German website, which is heavily reliant on javascript code unsearchable by Google, used text-heavy pages liberally sprinkled with key words to attract the attention of Google's indexing system.