I don't think it's a good reason, and FWIW I'm pretty sure they got the idea from C++ - iostream design predates ISO C++ by quite a bit. I remember seeing << for that with stuff like Borland C++ 3.1 that also originates in 90s, back when you had to write #include <iostream.h> etc. Just noting that this is not such an obviously bad idea that nobody else hasn't fallen into the same trap.