Is "net/http"'s use of global variables considered a good practice in golang?

The golang package "net/http" uses the global variable DefaultServeMux to register handlers. Is this considered a good practice or even an golang idiom? Is it a global variable after all?

The two main reasons not to use global variables are AFAIK 1) that they add to complexity and 2) are problematic in concurrent programs.

Maybe 1) is not considered important in this case because the developer can choose not to use DefaultServerMux? What about 2)? Are global variables always thread/goroutine safe in Go? Still, I'm surprised that it's used in Go's standard library. I've never seen such practice in other languages / standard libraries.