Actual arbitrage is legit, the bots probably aren't.

Arbitrage is just depositing X amount of n coins on one exchange and the same amount of X n coins on another.
When one drops down in market price, an order executes to buy at the lower price and another order is Plas ced on the other exchange to sell at the higher price so uou still end up with the same amount of funds. You or the bot will then have to withdraw the difference to make the accounts the same again.