The spec could contain, "don't add any bugs into your C programs" but that doesn't magically mean that everyone would write bug free code.
Reserving a namespace and not having a mechanism to enforce it was a massive fail on the part of the standards committee.
How do you expect the compiler to enforce the "identifiers that begin with two underscores are reserved for use by the implementation" rule? Some things just belong in the documentation.
Problem solved. Instead, they did something awkward by trying to carve out namespaces for the stdlib too but quite frequently don't adhere to their own rules.
Go and read your /usr/include/stdio.h (or equivalent). It will be filled with such symbols. Obviously the compiler must lex them exactly as it would unreserved symbols.
This is an unfortunately legacy of C's decision to pass compilation information around by textual inclusion, and isn't really fixable at this point.
I do think compilers ought to have warnings for using identifiers in the reserved namespace. Most projects probably wouldn't trip over the ^_[A-Z] reserved space; however, numerous projects would trip over the reservation of any identifier containing two adjacent underscores.
Conceptually a warning is a nice idea, but it just isn't practical.
The problem is that too many people assume that their usage of C is the same as everyone elses. I don't want to be writing a system library and fighting my compiler everytime I put something in the symbol table that isn't in the C spec.