The process absolutely works both ways. Arbitrage does not drag the price down or up. Its sole function is to correct inefficiencies in the wider market by tying various markets for the same asset together. An arb trader simply finds traders on one market and connects them to another market where they could have themselves bought or sold and got a better price if they knew better or were willing to go through the trouble(sometimes involving 2 or 3 other assets). For example if the price on poloniex STEEM shoots up, that price does not reflect the full market because not all orders/traders are on poloniex. The buy orders must fist reconcile with all the other markets where traders are selling STEEM either for BTC or SBD, or any other asset. The arb trader simply makes those connections. Arbitrage does not effect the real price, it helps to equalize the price across all markets to establish and find the real market price, where all traders are accounted for. Only when there is no arb opportunities left can you say you have the real market price.

You've likely been seeing this one way action because that is simply the direction the market has been going. There is a good deal of sell pressure from STEEM that needs to work its way out of the system through both STEEM/BTC and STEEM/SBD/BTC.

Anyone buying SBD and then converting using the SBD 7 day settlement function is not technically doing arbitrage. They are speculating and risking capital on a bet that after 7 days the price of steem will be to their advantage. All markets are subject to this type of speculation.

There is no speculation in arbitrage. I hope this makes sense : )