Without the third guy standing there with a club it's not clear if or why they would demand dollars.
Well, maybe sales taxes when he spends his ill-gotten gains, but that's indirect and if he's effective at intimidation with his club, he arguably doesn't need the money in the first place as he can acquire goods through other means.
The more compelling argument is that we pay taxes primarily to enforce peaceful law and order, primarily through control of force - at home through the police and abroad through the army - so that our stuff isn't just taken by "someone with a club" and making it worth being part of a larger society.
You could argue that government isn't much different to a mafia, except that modern taxation is generally more transparent, fairer, impartial, and perhaps more considerate to the people with the least money. Also, they generally don't use force to enforce taxation, only the threat of a prison sentence.