Windows: Why does Bonjour's mdnsNSP.dll inject itself into every process?
Sounds like you are curious about security and privacy. If so, your question should be "Why does my operating system allow third-party software to become aware of and affect every process?" or, "If I care about security and privacy, why am I using Windows?" 

Bonjour is Apple's implementation of zero configuration networking. A very long time ago, and very shrewdly, Microsoft began making its OS and most of its processes, and the file system as well, network aware. Most notably, the file manager Windows Explorer was made network aware, which was also basically the same application as the native web browser, Internet Explorer,  and this change in the behavior of the operating system led to antitrust cases regarding which web browsers shipped with Windows. By now, I imagine, everything running in Windows is probably network aware, and are identified and advertised as 'Services' to the Windows kernel and the network. If so, it makes sense that Bonjour would make itself available to every processes to enable zero configuration networking.