[Piece of Cake]

1. Money, money, money. More tax revenue from taxes. More revenue for domestic corporations if they can enter the market, instead of the current situation were nondomestic companies are making a killing off of US consumers.
2. Prohibition doesn't work - see alcohol.
3. Prohibition doesn't solve the problem he is trying to prevent. He doesn't want addicts and underaged playing and doesn't want it to be used for money laundering. Forcing poker underground doesn't stop the abusers, it only stops the law abiding users from playing, domestic companies from profiting, and the US government from effectively taxing and regulating the industry. Legalizing and regulation allows for implentation of means to prevent abuse. The UK sites have done a number of things to this effect.
4. Freedom of discretionary income. If I want to spend $400 on a bottle of wine, or a round of golf, or a game of poker, it's my choice. It's entertainment.
5. Separation of church and state. Gambling is a form of entertainment paid for out of discretionary income. One's religion may prevent them from partaking in the activity, such as it does one from drinking alcohol. But our country was formed on the freedom of religion.