Why does ((unsigned char)0x80)

C compiler performs integer promotions before executing the shift.

Rule 6.3.1.1 of the standard says:

If an int can represent all values of the original type, the value is converted to an int; otherwise, it is converted to an unsigned int. These are called the integer promotions.

Since all values of unsigned char can be represented by int, 0x80 gets converted to a signed int. The same is not true about unsigned int: some of its values cannot be represented as an int, so it remains unsigned int after applying integer promotions.