Here's my wild speculation on why Musk did this (and why it's brilliant)

He knows there is a large and growing community of Bitcoin speculators and enthusiasts out there. By "endorsing" Bitcoin in this way, he not only moved the market--further enriching these folks--but also signaled that he is an ally to them, significantly boosting positive sentiment toward him and increasing the likelihood that they will purchase a Tesla as their next vehicle.

In one fell swoop, he not only drove demand for his product, but also made it more affordable to his target market without lowering its cost.