Indeed PBOC investigation was to look into possible market manipulation according to the statements posted on the People's Bank of China's website. But the reality china still have any impact for bitcoin and 'playing' with bitcoin. Your opinion isn't fully true if said that china are NOT HAVING power.
Manipulation and everything was being caused by high frequency traders exploiting the zero fee platforms, and thus could recklessly make the price go up and down to their heart's content. In fact, it made them look far more important than they actually were if you look at the situation now. Chinese exchanges are now nothing more than what western exchanges are. I even tend to say that they are
less than western exchanges, since a few of them (previously the undisputed nr1-2 with millions
BTC in volume every day) fail big time to generate decent volumes
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