shift jis is not true extended ascii. besides replacing the backslash with the yen character, multi-byte characters can also include ascii bytes. it does avoid the use of ascii delimiters and controls, so in many cases such as html it can work. utf-16 is even less extended ascii because ascii characters are stored as two bytes with one byte equal to 0x00. porting an existing system to support character sets as shift jis or utf-16 is complicated and bug prone.