 Why do Harvey and Joe change thier plan when confronting Johnson about the water?
When Harvey staggered pontifically into the battered metalloy saloon—the only one on Planetoid 42—his tall, gangling partner was already stumbling out, mouthing something incoherent. Paying no attention to them, it was carrying a case of liquor in two hands, six siphons in two others, and a broom and dustpan in the remaining pair. The interplanetary con-men drank noisily, asked for more, until they had drunk eight glasses. Our condenser broke down, and we've gone without water for five ghastly days." He came near starving and moved on to Titan. Water's five buckos a glass. It's bitter as some kinds of sin before it's purified. "Friend," said Harvey, taking out a wallet and counting off eight five-bucko bills, "here is your money. What's fair is fair, and you have put a different complexion on what seemed at first to be an unconscionable interjection of a middleman between Nature and man's thirst." "And chief of police, no doubt," said Harvey jocosely. "On account of the quantity, I'm able to quote a bargain price. Shucks, boys, it hurts me more to charge for water than it does for you to pay. The planetoid man worked the pump while the mayor intently watched the crude level-gauge, crying "Stop!" when it registered the proper amount. Then Johnson rubbed his thumb on his index finger and wetted his lips expectantly. Johnson's response almost floored them.
 Joseph's son is large and intimidating, and they want to avoid a fight.
 Joe suddenly feels unwell, and Harvey needs to help him.  They want to buy Genius, and don't want there to be bad blood.  Joseph's son is large and intimidating, and they want to avoid a fight.  They don't think they could take Joseph in a fight.