That can't be the reason since including a C header as <stdlib.h> is guaranteed to dump the identifiers into the global namespace (but can additionally put them in std). Including it as <cstdlib> is guaranteed to put the identifiers in std (but can additionally dump them globally).
A little inaccuracy saves tons of explanation. - H.H. Munro
Oh, I see what you mean. I must have skimmed the part about including cstdlib vs stdlib.h. In that case, yes, it might be to ease compiler implementation, or it could be written that way in the standard because existing compilers do it that way. It's basically a warning to the application writer that those function names might be defined at the global namespace, so the application writer cannot use those function names.