Making realloc with a size 0 be undefined is a way of maintaining existing code, because it means that different implementations kan keep doing what they are doing. Some implementations free the pointer, some create a zero size allocation. Since there are code depending on both behaviours, we cant define a consistent behaviour, without breaking code that depends on another behaviour. This way we acknowledge the issue without breaking anything.

There was a discussion if it should be defined as "platform defined" instead, but the wg14 wanted to indicate that they discourage this use of realloc, and undefined behaviour was a way of doing this.