I'm just amazed they allow zero-deposit transactions with such a low fee.
Obviously they did in OP's case, but their terms actually say that they don't:
If the fee is too low, your deposit will not qualify to become an instant deposit.
I wonder if it was a case of their algorithm making a mistake, or maybe OP is a long standing customer with lots of deposits and account in good standing and so he gets the benefit of the doubt. Even when the transactions were broadcast back in February, they were still around 80 vMB from the tip. I can't imagine this is standard practice for Roobet, because as you say, it becomes trivial to double spend the transaction back to yourself if you lose.