I believe that you have to declare carrying over $10,000 in cash. It is not illegal to do so, but regular cash carrying trips will get you noticed by US Customs and the DEA I am guessing. You will probably come in the spotlight and maybe selected for an up-close-and-personal encounter with an immigration official (the kind with latex gloves). I am not sure about this, other than the requirement to declare over $10,000 in cash.
The land of the free is not so free anymore.