This goes all the way to early BASIC, and it's signed because the language didn't have any unsigned numbers to begin with. The main reason for this particular

This goes all the way to early BASIC, and it's signed because the language didn't have any unsigned numbers to begin with. The main reason for this particular arrangement is that, so long as you can rely on truth being represented as -1 - i.e. all bits set - bitwise operators double as logical ones. Thus BASIC would have NOT, AND, OR, XOR, IMP, EQV all operating bitwise but mostly used for Booleans in practice (it misses short-circuiting, but languages of that era rarely defaulted to it).