Has anyone ever written a paper and put your name on it? I am a professor and researcher in mathematics and my research is kind of on the theoretical side.  Online, I discovered a mathematician who had written some papers with ideas similar to mine, and we corresponded and discussed possible collaboration.  Then he wrote an entire paper, listing both of us as authors on the front page, and sent it to me.  I didn't think it ethical to take any credit for a paper I had no part in writing, so I discontinued all contact with this person.  Has anyone here had a similar experience?  How common is this?

I am becoming very disillusioned with the pure research game.  People spend huge amounts of time on self-promotion because there just isn't that much demand for their work.
Without knowing too many details of your situation, could you perhaps give the other author the benefit of the doubt? Cutting off all contact does seem harsh (unless you have other reasons) for what could simply be a misunderstanding.

I don't know the conventions of your field, but it seems in many science fields that authors are added if they contribute intellectual ideas on the research, not necessarily for doing the research. Other authors simply add support in some fashion, so get added. It is not uncommon to have many authors on a science paper. It does in fact get tricky because one can offend others by not including co-authors. 

I'm of the opinion that it's probably better to include someone who contributed in some fashion than not include them. You could argue that this dilutes research and is a "pure research game", but honestly, one paper is not going to make or break anyone's career.

Could this author simply have been trying to extend kindness, or perhaps thought you had a bigger role in the research than you actually felt you did?