This was a specific design decision, not due to lack of ability to implement a warning around. You can use acid(1) on the broken proc to get the exact source line that the code crashed on. Which can help narrow down the cause of a segfault.

It is standard C as far as I know. gcc with -Wall (at least on my machine) does not generate a warning for any integer constant to pointer conversion. The plan 9 compilers will generate an error for all integer constants to pointer conversions other than zero. So we are actually more strict than UNIX compilers about this! It is legal to define NULL on UNIX as just 0 (without the cast). So my guess was that this exception was kept for compatibility with UNIX code.