Unfair! I was expecting to get lambasted to the 13th generation, instead you've confounded me by reasoned argument. As far as I can see, most agree that the use of the "sensei" honorific in non-dojo situations seems to be determined by a combination of familiarity and/or cultural imperatives. This seems emminently reasonable. However, I still find myself unhappy with the thought of using the term outside the dojo, but I'll put that down to my own background and culture.
What I'm now curious about is how people who are actually referred to as sensei by the students outside the dojo feel about their students doing this? The only time I ever experienced this (a more incorrect use of the term I can't imagine I hasten to add) I felt only acute embarressment.