It's not a bug. The order was most likely executed to repay a debt brought about by borrowing XRP to margin trade. And it is not like he didn't have to pay any BTC to make the purchase. It is just that the amount required to buy that XRP was less than a satoshi, so it reads as 0. When you do enough of these sub-satoshi trades to add up to 1 satoshi, Poloniex make you pay it.
I hope you are right, as I do not do margin trading. At first, I thought it was a bug and thank you OP for bringing this observation. Is there someone can confirm sir @Hatcher explanation? Also, hope poloniex staff can clear this issue.I am also trading in Polo everyday.