This is just a "triangle" arbitrage which has exactly the same results. You would sell steem for BTC, Buy steem for SBD, and Buy SBD for BTC. You are both buying and selling each of the 3 items. Again in all of these examples the orignal buyer could do all of these trades himself to get the best price but he doesn't because he is either too lazy, unwilling, or unable. So the arbitrager does it for him. These are all legitimate markets and you cannot say you know the true price of something until you considered all of them. Thankfully you don't have to consider them. The free market does this on its own. But anyway, the price cannot move up or down until it exhausts the orders on the books for all these markets.

If I have a banana farm and there is some dude who has lots of Steem and loves bananas. We can trade Steem/Bananas until I am out of all my bananas. The fact that he sold them to me in exchange for bananas means there was less selling pressure on Poloniex. This is just to illustrate that every market needs to be considered if your trying to determine the real price. I can also trade Steem for Gold, Steem for silver, Steem for Ethereum etc etc. The fact that these other markets exist is not sabotaging the price of steem. They are all elements necessary in order to determine the real market price.

If i can buy more bananas with the steem I earn from selling them. I will continue to do so until the market price of bananas increases too much from my buying them all, or until the steem i can get from selling bananas is too little to replace the bananas i sold. There are millions of markets like this and they will all effect the ultimate price of steem (and bananas)

Sorry for the somewhat silly examples. It gets complicated to describe pretty quickly and evidently I'm not finding an easy way to say it. : )

Here's one last attempt: If there is a buyer for Steem with USD at $1.50 and he has an infinite amount of USD. If you can buy Steem with $1.40 worth of gold. or $1.30 worth of pumpkins, or $1.35 worth of Ethereum, or $1.38 worth of BTC etc etc... Someone will do all those things and sell the Steem to this infinite USD buyer at $1.50 until the price of steem in gold is $1.50 or more, the price of Steem in pumpkins is $1.50 or more, the price of Steem in Ethereum is $1.50 or more etc etc. This is just the market. How it equalizes. How price discovery works.