Compiler optimization are allowed to change how often a pure function is called, and you can freely change it in your code. If it had a printf inside it, you'd need a way to say you don't care about the side effect of printing a specific line one time, and not zero or two times.

Of course there is lots of code that has side effects in implementation but not in their interface. Like malloc, or a read only data structure that has an internal cache it updates.