Users wouldn't be prohibited from doing this for much the same reason people aren't prohibited from making personal copies of pages from books they own.
If you take a picture of Google Maps and then host it on your website without approval/paying Google and get caught you'll be hearing from their copyright lawyer.
I could take a picture of a map and share it on my LAN via httpd so all my computers can access it. I am the only user on the LAN.
It is not the "website" aspect that would implicate copyright, it is the redistribution, e.g., via a website on the public internet.
My original question is being misunderstood. It is not about copyright or what rights Google has in maps. It is a question about why Google attempts to force users to contact them every time the user looks at a map.
If you take a picture of Google Maps and then host it on your website without approval/paying Google and get caught you'll be hearing from their copyright lawyer.