Charitable interpretation may be: Back then when the contract of this function was standardized, presumably in C89 which is ~35 years ago, CPUs but also C compilers were not as powerful so wasting an extra couple of CPU cycles to check this condition was much more expensive than it is today. Because of that contract, and which can be seen in the example in the below comments, the compiler is also free to eliminate the dead code which also has the effect of shaving off some extra CPU cycles.
