One reason is that it is spreading the idea of FOSS. Because the great thing about JS is that it can be distributed in only one form: source. Since all your code is visible to everyone anyway, you might as well publish it under a libre licence. And that’s what most people do.
It’s also nice to see new-age hackers doing all sorts of spiffy stuff at npm, the namespace pollution mechanism. What’s more, thanks to npm, this new generation of developers is carrying forward the UNIX philosophy:
Do one thing and do it well.