If you were to post an empty message on here (no text), would it be off-topic? :)

Going back to the question:
I don't think missionaries hurt foreigners. I think Chinese are smart and stubborn enough to make up their own mind. I think that if student are annoyed by a teacher or complain about him, he is probably hurting himself more. And in the end if it progresses for a long time may lose students and/or his job.

About missionaries:
Of course if one is religious it may pop up in a conversation or if teaching subjects like sociology, American studies, History, Religion, Politics and Public Policy.
I think there is a thin line they should not cross and should be especially considerate of free will.
Inviting students who are interested, I think is fine if it's already part of their class. Don't teach PE and ask students if they feel like reading your book after class.
Teaching your religion without authorization / informing students prior is definitely wrong. It could even spark heated arguments and fights. (I remember very well pulling an old lady off a guy as she was trying to bash his head in with a beer glass, simply because he laughed at her preaching in a pub full of drunk people. At which point she started to curse everyone like an old witch).
Think of it like this: when someone wants to go to Church he'll go in himself. You don't have to drag him through the gate or put the Church over his head.
Unfortunately "free will" is very subjective in many people (especially hardened or extremists' minds). I wouldn't be surprised if people who traveled in Pakistan would have been harassed to enter or learn once they raise the topic of religion. A simple question out of interest there can put you in a true predicament.

Another thing that I think should be kept in mind is: level of interest / like-mindedness. Someone might already have questions, but I think they should all be addressed at school. Not at someone's home. If after a while someone wants to really focus on it and learn it, then why not? Better to ask your teacher in my opinion as he can probably answer your questions better.
My concern always is: what's the missionary's background. How fanatic is he/she. Some are like wolves in sheep clothes, others are awesome teachers and truly only focus on the language.

LOL, that's funny you mention foreign students.
There was something in the news in the USA and Europe a couple years ago about a ton of corporate espionage and much more happening. They claimed, and even kicked some students out of what I heard who were "interning" at various companies. Apparently some were bringing back / sending back information to their true employers back home (mostly in Asian countries). That made a lot of companies tighten their application process and screenings a lot and restrict a lot of access to anything and everything that could be used by other companies.

* comment to the moderator: maybe it's time to close this thread or moderate it cause I spent nearly half my time reading arguments rather than solid topic contributions.

There were a number of things I loved in this including nnoble's comment about how qualified teachers tend to lean towards teaching and unqualified teachers tend to entertain. I have seen this happen.
And the Dudeson's comment about how this is a wonderful ground to experiment various teaching techniques.

Personally, I used to suck at French, but I had a wonderful teacher who was great at teaching. She added creativity to the class material (not entertainment). It engaged the students in thinking outside the box and apply what they learned, it helped me pass my classes.
To this day every time I have to teach I apply this system as I know just how hard it can be for someone to learn a new foreign language.