Arbitrage is supposed to be pretty much risk free, that's the whole point of it.
Being exposed to a trading loss means that one leg of the arbitrage is being opened or closed at a different time to the other, so effectively they would be "pair" day trading, speculating that the difference between two markets would increase/decrease in their favour over the course of a trading period.
That's not arbitrage, that's conservative day trading, which is presumably why they refer to a 3% stop loss.

On their FB site they say they have $2,000,000 available for trading, so I guess that means they have to give a return on that $2m whether or not they use it all. They also have to pay their overhead.
0.5% per day on $2m is $10,000, plus somewhere I think I have read they take 20% for overhead, so that means to return 0.5% they need to gross $12,000, to return 1% per day gross trading profit $24,000 etc.
Say they can make a $2 per BTc inter exchange arbitrage nett profit after spread and finance costs, that would require a daily trading volume of 12,000 Bitcoins, turning over their $2m about four times, in order to return 1%.