Is it a law or just industry protocal that all slot machines are in the key of C?

It is not a law that all slot machines must be in the key of C; rather, it's an industry convention. Slot machines often use the key of C because it is a simple and familiar tonal center that appeals to a wide audience, making the sounds more engaging and pleasant. However, manufacturers can choose other keys for their machines, and variations do exist. Ultimately, the choice of key is influenced by design preferences and marketing strategies.