Regardless of the current state of their economy, they have the money, and money is power in today's world - money talks, merit walks.
On the small matter of national laws, how many US servicemen, accused of rape while serving overseas ( and there have been quite a few in the last fifty years or so) have ever faced trial in the country of the alleged offence, rather than being shipped back quickly to the US?
I'm not sure what any of that has to do with Barclays agreeing to pay a fine.
What has happened is that between 1995 and 2006 Barclays staff in New York forged documents to hide transactions with countries which are on the US governments 'banned or restricted commerce' list. These are countries which the American government has identified as posing a risk to America's security, foreign policy or economy under the terms of the International Emergency Economic Powers Act.
This is a Federal Act which empowers the President to declare an embargo of a specific country, or to closed down organisations during a declared state of emergency. Since 1933 American Presidents had almost limitless power to declare emergencies, and to prolong their existence without asking Congress. In 1973 this power was revised and limited by the new Act I mentioned above.
Barclays admitted that it acted illegally by concealing commercial transactions with banned countries, and has agreed to make substantial payments by way of fines. Barclays isn't alone in concealing its illegal dealings - Last year Credit Suisse agreed to pay $538m for hiding thousands of transactions on behalf of clients in Iran, Sudan, Libya, and earlier this year the Wells Fargo bank agreed to pay $160m for failing to stop more than $100m of Colombian and Mexican drug trafficking cash being laundered through its accounts.