It’s been illegal to proselytize door-to-door in Cambodia for years now, ever since an edict in 2003, passed down shortly before I arrived. Not that it actually stopped anyone. As Details Are Sketchy notes, you could always see the LDS (Mormons) Missionaries on their bikes – and the Mormons are only the most iconic of the group. At least most of them actually speak Khmer quite well. The majority of Christian missionaries in Cambodia are frankly not doing much good, but seem to be living out their petit-bourgeois dreams in Cambodia while stroking their moral compasses. Some groups, however, are in fact doing excellent work out of a religious motivation – but these are precisely the groups least likely to care about a ban on proselytizing, since they don’t do it anyway.
And anyway, Christianity has made its biggest inroads among the Khmer precisely with the ‘former Khmer Rouge’ populations in places like Pailin. Replace a system that promises karmic retribution for action with one that promises the same thing, but then lets you know that all you have to do is ask for forgiveness and you get a ‘get out of hell free’ card. No wonder it appeals to that crowd.