It's a culture thing and a recruiting tool. Especially in the tech/new media industry, Campus style headquarters are very popular. The Googleplex, for example. It creates a "loosened" environment, encourages self direction and innovation (or is believed to do so at least), and enables rapid expansion.
It's not surprising that other companies would attempt to follow the behavior of some of the most innovative and successful companies of the past 20 years.
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