I don't think they are insane. It is safe if you follow some rules. Flag -Wformat=2 catches every mistake at compile time. (It wont guard against overflow of course, but this is C)
The last example is explicitly defined in the Standard.
The last example is explicitly defined in the Standard.
Implementing all that subtle functionality correctly takes up a lot of CPU time.
According to my quick test, on Visual Studio 2012, even simplest sprintf with just one parameter seemed to take about 1 microsecond to execute.
Of course clang and gcc seem to sometimes compile whole format parser away. At least...
printf ("Hello World!\n");
Of course iostream << operator runtime performance is also pretty horrible. Each << invocation seem to call streambuf::sputn (or sputc) separately. On VS2012, simplest stringstream test...
ss << "value: " << intVal << endl;