This is not the prisoner's dilemma table. The Nash equilibrium is the best possibility for both. When a player chooses S, he helps the collective but also helps himself selfishly. There is nothing anyone can gain by "defecting". Every miner chooses S because it benefits him, not because he has some sort of pact where the other miner does the same.
The defector does gain by defecting; They gain market share. This isn't a prisoner's dilemma, it's just the best way for a mining entity to out-compete the other. It doesn't make a lick of difference that they can make more profit (
Split between the two) by cooperating over a long period of time by not leveraging a competitive advantage over that same period.