They don't have the required control over what they do. American Express don't want to deal with porno sites anymore since there are too many problems and refuse all payments. All kinds of computer attacks already happened. But there are many more attacks of all kinds. Even DNS servers have been attacked.
Crackers enters computersystems of big companies, research centers, gorvernements or armies.
They stole or destroyed information. This shows the vulnerability of this networked world. I am not a financial analyst. Neither do I have the knowledge of stock exchanges. As I understood of my readings, the internet offer the service to easily buy and sale stock options.