You have to consider the full specs.
First of all:
Expects: pos <= size().
If you dont follow the precondition you have undefined behaviour anyhow. Now…
Returns: *(begin() + pos) if pos < size(). Otherwise, returns a
reference to an object of type charT with value charT(), where
modifying the object to any value other than charT() leads to
undefined behavior.
The only (valid) case that “otherwise” refers to is when pos == size(). And that is probably to emulate c string behaviour that have a some_string[size] element that can be accessed. Note that charT() is typically just '\0'.
PS: One might think that to implement the specification, operator[] would have to check if pos == size. However, if the underlying character array has a charT() at the end of the string, then you get the described behaviour basically for free. Hence, what seems a little different from “usual” access into an array is actually just that.