This isn't that simple when using arbitration with multi-signature and not an escrow. The company providing the arbitration services doesn't have full control over the funds, and cannot do anything with it without the agreement of the buyer or seller.

To pull a runner, they would have to get the buyer/seller to agree to participate in the fraud. This is not simple to do in a large scale as it requires them to contact every buyer/seller and ask them to commit fraud, which could easily get published publicly by someone honest that was contacted by them. When it becomes public, the buyers/sellers can just move the funds to a new multisig with another arbitrator.

And besides all that - a popular and reputable arbitration company could be making good money from just acting honest and providing arbitration services. I don't think they would destroy their reputation for some quick buck.