The reason for this is given in n3512 string_ref: a non-owning reference to a string, revision 2 by Jeffrey Yasskin:
I also omitted operator+(basic_string, basic_string_ref) because LLVM returns a lightweight object from this overload and only performs the concatenation lazily. If we define this overload, we’ll have a hard time introducing that lightweight concatenation later.
It has been later suggested on the std-proposals mailing list to add these operator overloads to the standard.