In a relatively unregulated and globally-active market, why would the US spearhead the probe into illicit trading? This means they already have interests on the said market and are looking for ways to tax the exchange operators hard alongside the users. What I want the US to do is a major crackdown on illicit ICOs as they're the ones who's causing major trouble in the cryptospace right now, and market manipulation will always be there no matter how many investigations and probes they'd do. Also, have a peek on traditional stock markets and see whether market manipulation is present or not.
They obviously had interest all along, and we know how much they kick against illegal and illicit trading right from the onset. Although they made a valid point, that a manipulated market by investors will end up being perceived wrongly, and let's face it, that is not good.
I know how much manipulation of some cryptocurrencies can be so annoying as a trader just because some whales always see it as an opportunity to use pattern and volume to deceive traders into getting a position and these days it has become a huge risk to trade the cryptocurrency market, but like you said anyway, it is a decentralized market, so all these kind of stuffs are bound to happen.