Even if you were able to get the taxes owed on the money stashed in HSBC's bank accounts, you wouldn't get it next year or the year after that because the owners would move their assets to another location.
It's a noble idea--make tax cheats pay their fair share. In theory, we could close budget gaps. fully fund education, and put free sweaters on puppies everywhere. This is the goal of tax collection agencies throughout the world, but it rarely turns out to be practical in the long term. You might see a surge of revenue one year and then the tax cheats find a new haven for their wealthy.
HSBC shouldn't even be allowed to do business in the United States. Withdrawing the protection of the American rule of law would force many wealthy people to go off shore. If you think you can get a better deal paying European tax rates, go live there. Wait until some shady organization with the legal backing of the state confiscates your wealth and then try to get an American lawyer to sue to get it back. I will you well in that endeavor. Ask a former Russian billionaire how they got their money back from the government. I'm still waiting to see what that looks like because it would be fantastic and wonderful to behold.
People are always going to try and cheat, even though the smart money is on using America's legal system as a guarantor of perpetual wealth. Once they have been robbed by questionable governments and sleazy bankers outside of the protection of American courts, let them come home and live under the law and pay their fair share of taxes. Got a bank in the Cayman Islands? Good, steal from these people so we can laugh when they come begging for the protection they saw fit to abandon.