Why do shift operations always result in a signed int when operand is <32 bits

Why do shift operations on unsigned ints give an unsigned result, but operations on smaller unsigned operands result in a signed int?

int signedInt = 1;
int shiftedSignedInt = signedInt << 2;

uint unsignedInt = 1;
uint shiftedUnsignedInt = unsignedInt << 2;     //OK. unsigned result

short signedShort = 1;
int shiftedsignedShort = signedShort << 2;

ushort unsignedShort = 1;
uint shiftedUnsignedShort = unsignedShort << 2; //CS0266: Can't cast int to uint

sbyte signedByte = 1;
int shiftedSignedByte = signedByte << 2;

byte unsignedByte = 1;
uint shiftedUnsignedByte = unsignedByte << 2;   //CS0266: Can't cast int to uint