There will always be people defending such schemes until they shut down. Look back at Pirate (for example), there were plenty of die-hards that defended him until he shut down and didn't pay out. Maybe these people just want to help perpetuate the fraud or maybe these people are really just that naive.
As long as people are greedy and fall for scams, the scams will continue to exist.
In this case, much like other similar "investments", a simple question is "If they can make as much as they say, why do they need our money?" If they are really that good, why wouldn't they fund their own operation and keep the profits for themselves?
Why the fuck do you even care if I loose my money here or not?
Some people do not like seeing others fall for an apparent scam or they do not want to see the operators to become unjustly enriched.
Because making a large profit off of arbitrage requires both liquidity (provided by the exchanges) and a large amount of capital (provided by either the business or investors), and they may not have that much to invest on their own. With other people investing, they make significantly more profit. If you have 1$ to buy gold on one exchange, to resell for 2$ on another exchange, that's great. But you still only make a dollar. Now, if you had 1 dollar and 99 other people to invest 1$, suddenly you've made 200$ off the trade, and you share out the profit to the investors, plus a cut for yourself because you took the time to find the arbitrage opportunity and perform the trades for the investors.
I am not convinced one way or the other, but from a pure business perspective what they're doing does seem viable. If I had the capital myself I could easily make 1% on arbitrage every day, but it requires you to keep both USD and BTC in large amounts on several exchanges so you can react to new opportunities without waiting for cash to transfer everywhere.