Ethically the entire thing sounds a bit dicey to me (for any religion, thank you very much), but in this case, even more so, since the people claimed on a VISA document that they were there to be teachers. If they ARE there to teach, and are legitimately teaching english, then their pay should be theirs, not any religions. That they did go there under the false pretense is wrong from the get go, but to have to give up their wages to the church seems to make it an even bigger lie-so that their pretense for getting in is beyond that they are teaching (and 'witnessing on the side), but that their ONLY purpose then becomes to be a missionary-which is NOT what they claimed on their Visa. Being a dedicated christian who teaches english is one thing, but using the visa process to lie when your ONLY reason for being there is to be a missionary (ie everything they are doing is strictly at the behest of the church, and they have no freedom to do anything of their own) seems to be lies on top of misleadings.
If ALL they are doing during the teaching is proselytize, they have no right to be in the country at all. They require teachers and need them for real or they wouldn't allow them in. That should be a separate thing from any missionary activity.
That being said, many missionary groups go in under these false pretenses, but at least MANY of them actually teach or provide medical care or agriculture assistance or what have you.