getopt defaultGetoptPrinter is not safe. Is there a reason for that?

[Markus]

Hi.

I'm a big fan of @safe code. But within my first 20 program lines 
I already run into problems:
Error: @safe function D main cannot call @system function 
std.getopt.defaultGetoptPrinter
https://dlang.org/phobos/std_getopt.html#.defaultGetoptPrinter
I wonder what's the reason for that?

Cheers
Markus