I agree with you that the original premise of Polo being in on the wash trading is unlikely, but I wasn't suggesting that at all.
You actually believe the Eth wash trader is paying 100-200 BTC per day (sometimes probably even more than that) just to wash trade? It's obvious Poloniex has made a backroom deal with them because they'd be better off just buying the actual coin to manipulate the market rather than wash trading if they were paying that much. The jig is up on this scheme.
Like I said, large traders might get more discounts than the public fee schedule. That doesn't mean Polo itself is wash trading, plus as tokeweed pointed out, other exchanges trade a lot of Eth too. Unless they are all conspiring, they're not in on it. More likely if it is going on, the people involved are independent of the exchanges.