They don't care about deficits. It all makes sense when you look at it through the extremist lens of far-right libertarians. 

Basically, they want to cut taxes without filling in for revenues. They want to force massive cuts by intentionally causing a funding shortfall; thus, reducing the size of government. They don't really care who it hurts in the process or what quality of life those services provide for most Americans. Things like parks, roads, clean water, etc are all something better managed by private owners - government is the enemy & their fix is to prove it doesn't work through deliberate sabotage.

It's a perverse way of thinking, but to some of the more gullible and idealistic among us it's the perfect solution. Who cares about collateral damage when they have a chance to start building their utopia on the altar of Ayn Rand's books?