https://bitfloor.com/ which is not a particular shady site (in fact, apparently they're already registered with FinCEN) allows you to buy BitCoins by depositing cash into their Bank of America account. There's an upper limit, of course, but there's no ID required, so nothing stops someone from driving to multiple Bank of America locations.
If you don't attract attention, then yes, "nothing stops someone from driving to multiple Bank of America locations" but intentionally doing that to skirt the limits actually is prohibited - it's a crime, just not as easily detectable.
If you do it in a large enough amount or you become suspected of some other issue, monitored and this comes up - then you'll be prosecuted for that.
But the point is that it would be pretty easy to do it without getting caught. You could send multiple people to multiple banks, across a wide area. The cost of manpower and fuel would eat a percentage, sure, but presumably any form of money laundering does. Which explains why the feds are interested in BitCoin.
Sure, it is not hard to do money laundering, and the risk of getting caught isn't that high - but my point is that even if you technically stay below the reporting limit, you're still facing prison if you get caught in the end; so it's not a trivial decision to make.
