It's highly concerning that apparently Bitstamp is showing willingness to manipulate the market given the role they play., because we don't know if there is any real foul play behind the scenes. MtGox once did the same with a similar statement in a press release. I deemed them somewhat more professional than MtGox, but perhaps I was wrong.
At least MtGox have stated that they would disallow their employees from insider trading, but I have not heard anything like that from Bitstamp.
You can bet your hat there was a huge amount of "insider trading" on Gox, and that currently that's happening on Bitstamp too.
Do not forget people running those service are a bunch of amateurs (at best), which are not compliant with current financial regulations - to operate as an exchange in the EU you need to have a banking license, in most of the US states you need to be registered as a MSB, etc. - Bitstamp is operating with a SINGLE corporate account where they mix all customers funds, and probably their own too. Taking deposits without a banking license is illegal in the EU, and it is also very dangerous for the customers: if ONE Bitstamp customer is investigated the relevant authority will have no other option that to freeze the ONLY bank account Bitstamp has, de facto screwing up all their customers. That wouldn't happen if they were compliant and thus acting as a bank, setting up a separate fiat account for each one of their customers.
From a legal point of view, currently Bitstamp is the sole owner of the corporate account in which customers are depositing their trading funds, which is very bad for their customers and totally non-compliant, as customers are not exchanging their money for a service but DEPOSITING it in order to trade with each other.
You can bet Bitstamp knows this very well, and yet they continue to operate because the risk is worth it - you can bet they are ready to take other kinds of risks too as per "insider trading".
That said, while operating an exchange without a banking license (or a banking partner providing that license as Fidor Bank does for bitcoin.de, which from a legal POV is just a tied agent of Fidor) is illegal/non-compliant in the EU, Bitstamp is not giving a shit about that fact risking all their customer funds to LE or regulators, I bet the apply the same standard for other dangerous practices.
We are still in the wild west, folks... But probably not for too long.