Since EOS launches its coin on Ethereum platform, they own ETH in billions of USD value.
They can play games with it and influence the price of ETH if they want.
What do you think they will do with this amount of ETH? Is it not dangerous situation for the ETH?
I remember reading somewhere that they hold about 3-5% of the entire ether supply, this combined with the amount of ether that icos currently hold could be a massive time bomb for ether, right now a lot of these icos are choosing not to sell thier ether because many of them feel that they can get more money holding it (essentially they are speculating on it/another story altogether) and in case of a market crash you might find a lot of them will want to dump to much if it to quickly which could amplify any crisis.
As for eos itself yes they could potentially use it to attack and destabilize ethereum once they release their mainnet and become their own blockchain, but if they try and dump everything together than it will only serve to lower the value of the ether they themselves hold.
I personally dont think they have anywhere near that amount of ether, i feel they have just been using the ether they raised during the initial few months to buy into their own supply on a daily basis(maybe a massive money laundering operation/conspiracy theory).