Here is a thought, perhaps the reason why it is not allowed is that internet gambling cannot be regulated very easily. The logistical nightmare that is such is painstaking. You can potentially have any Joe Schmo in a foreign country open up a server/gambling site for the purpose of potentially scamming funds out of the US. It is easy for getting into gambling online and so I think many people will potentially partake.
Thus in perhaps a few years, you will see some problems with the economy in that it is another outlet for the country's wealth (it rests within the people, no matter what you think), to be unbalanced with the imports and exports.
The other fact why this is not allowed is that there is no way to even tax those businesses or keep accountability of any wrongdoing. There are rules that are set up with businesses for a reason, both foreign and domestic, in hopes that the whole world economy (from our perspective) doesn't put us at a disadvantage. Cry free market all you want, but without any rules and regulations to play by, be prepared to be scammed out of house and home (and that is one of the many many reasons why it is in place.)
You really want this country to be even more messed up than it is right now? Think and have some foresight as to what will happen if internet gambling (in its not fully cover all bases form) to be realized? Eventually it will happen but until we can make sure ALL bases are covered to where the US as a whole won't be screwed, then it will stay the way it is.
I am not also discounting the local gambling businesses also not allowing it either, it is bad for their business. But to say that is the sole reason, I just can't accept that.