There are several languages that have great similarity in names and some only differ by a character or few characters: C, C++, C#, Cω, Ch; X++, XL, XSL; Go, Go!, Goo.
A standard for trademarks is risk of confusion; as the "!" is not pronounced (or typically indexed), the collision between 'Go!' and 'Go' is much stronger than the others you list.
If the 'Go!' creators had any hope for using that name on a series of offerings (tools, books, etc.), they have to object to Google's use, to retain the presumptive rights they'd established so far via use.
Yes, I'm starting an internet company called "Yahoo", which shouldn't cause any confusion with that other company, "Yahoo!", widely known as "Yahoo Bang". Right, give me a break. :)
So why again should Google make the name change?