Holding a patent on an entire category of product -- like a car, or a legal simulation -- is very rare. Usually patents cover smaller details, such as the way that you represent legislation within your system, etc... I just brought it up as copyright and trademarks are not an issue for your example, only a theoretical mega-patent (which might not even hold up in court) could affect that situation.
This is true. Its usually the little intricate bits that have the patents. For example: If you have a stock market simulator there would be buttons for buy and sell which is fine. However if you print the bid and offer price on the actual buttons that the user clicks then this part is patented.
In your example of a legal simulation it could be that the US company has patented a UI interaction to allow a lawyer to create an objection. If you copied the same UI interaction without licensing it then this is where you would get into trouble.