I run a company with a significant cloud software component so I kind of do have some basic knowledge in this area, while you stated that you do not. In the interest of educating rather then flaming, however, I'll explain a little. If a regular user from China hits your server you will indeed see their IP address in your server logs. A firewall is simply a construct set up to screen packets before they get to your server, and you would see those IP addresses there as well and could also instruct your firewall to block any range of IP addresses.
However, it is trivial to use a VPN or proxy in the U.S. This is a computer that takes in a packet from anywhere in the world, then resends it to its destination. When it is resent, the packet has the VPN or proxy's IP address. If that VPN or proxy is in the U.S. It will have a U.S. IP address and there will be no way for the server's administrator or the firewall to know where it originated short of getting a warrant to search the proxy computer. Networks like Tor take this to another level with a network of thousands of computers around the world set up as proxy's, most private computers, with each packet randomly bouncing between several computers in a number of jurisdictions with no records maintained and again easy to ensure a U.S. or European last hop to get their IP address. Anonymizing systems like this make it almost impossible to track down the origin of any hacker who has very basic level know-how. Go ahead, install Tor on your computer, hit your server, look at your server logs, and tell me what IP address you see?
Suffice it to say it wouldn't be at all surprising if there was a person much like lylec305 in mgt at the law firm who was sure they were fine because they'd blocked overseas IP addresses at the firewall without having a real understanding of what that meant.