Confusion over sign convention for power
Because whoever drew the circuit is adhering to the passive sign convention, which states that active devices that source power have a negative power term, and passive devices that sink power have a positive power term.

Resistance is always positive, any real quantity squared is positive, and therefore \$RI_S^2\$ must be a positive quantity. Consequently, \$-12\cdot RI_S^2\$ must be a negative quantity; the negative is added to adhere to the convention.