I don't really think that zero fees could somehow contribute to price manipulation
They might (and likely do) contribute to more volume as well as narrowing the spreads, but I can't possibly see how they would make price manipulations easier to perform. If we take 0.2% trade fees as the basis, then at the price of Bitcoin equal to 1,000 dollars per coin, this percentage would amount to just 4 dollars (0.2+0.2=0.4x1000=4). We will just see the spread widening, but that's pretty much all. Manipulations usually happen in the range of dozens (or even hundreds) of dollars, so 4 dollars are actually nothing in this regard. Anyway, I would like to hear your arguments and explanations how zero fees could make them possible
One example is where you make use of the 'stupidity' of bots that will constantly pop up and place orders above your order, which you use to sell into. In that case you can scoop up profits of +0.10% in a matter of seconds. Usually these profits are peanuts, buts since tranding fees don't apply they are a more than welcome bit of profit that just take HFT a second or two at most. Do that x100/500 times a day and you'll be doing extremely well.
Another example is that they have no problems dumping the price down, where they fill up the buy side of the orderbook with mostly their own orders, and dump thousands of coins at once to trigger panic sellers and stop loss orders that will sell into their buy orders. After that they do exactly the opposite and fill the sell side of the orderbook with their own orders, and start buying the price up and have people buy through their sell orders.
It's insanely fast how they used to trade. I have seen several 10,000BTC orders being used to dump the price down and to lift it up like $20-$30 in CNY equivalent in a matter of a few seconds. If you just look over your shoulders and look back at your screen, it would appear that nothing has happened until you look at the charts.