The Venezuelans invited them in because they needed the investment capital, and the technology, to develop this area. The Orinoco Basin has enormous reserves, but its extra-heavy stuff, its not the good stuff everyone is used to dealing with. Chavez himself has re-assured them several times in order to get them to stay and increase their investments. Now he will pocket the fruit of their labor.
With $17 billion invested, they are hostage to their investment, and Chavez knows it.
The US government isn't going to intervene to help a private company, and Chavez knows that too.
In another era, companies in a similar situation would be talking quietly, behind the scenes, to anyone they could find who would be sympathetic to their plight, military officers, spooks, young firebrands on the make, and with that kind of money on the line, who could blame them? If some jerk was going to steal $17 billion from you, what would you be prepared to do to protect it?
Start nationalizing CITGO, any other Venezuelan business, assets and bank accounts in the US to compensate American investors.