I can tell you why Mt Gox has fewer problems with Dwolla than TradeHill. MtGox goes to incredible lengths to keep the bad guys away from their exchange: they forbid web sessions coming from TOR, they ask for proof of ID for users handling the most money (over $1k/day or $10k/month), they track stolen bitcoins and freeze Bitcoin deposits made to them containing known-tainted coins, etc.
As a consequence, the bad guys tend to operate on the less protected exchanges, such as TradeHill, and they get hit by Dwolla's chargebacks, coming mostly from hijacked/fraudulent Dwolla accounts...
MtGox's report of $5000 of Dwolla chargebacks is so small it could very much have been only 1 or 2 incidents. Of course Dwolla is going to have much more time communicating with their largest customer about 1 or 2 incidents, than investigating possibly hundreds of fraud cases involving a customer like TradeHill representing a small fraction of their business...
As a consequence, the bad guys tend to operate on the less protected exchanges, such as TradeHill, and they get hit by Dwolla's chargebacks, coming mostly from hijacked/fraudulent Dwolla accounts...
MtGox's report of $5000 of Dwolla chargebacks is so small it could very much have been only 1 or 2 incidents. Of course Dwolla is going to have much more time communicating with their largest customer about 1 or 2 incidents, than investigating possibly hundreds of fraud cases involving a customer like TradeHill representing a small fraction of their business...