All religions recruit to some degree, not just those considered cults. A very effective way is to not outwardly recruit, which is essentially what's happening here. OP has already stated that this girl was nice, not pushy and so on. In fact friendly enough that she's considered inviting her into her home with her family and friends.
By doing it this way the target percieves they have some measure of control over the situtation which increases their likelihood of listening to some religious information at some point, if only out of pity or guilt.
I can guarantee you that the photographer is well aware that some people will take advantage of the free photos without committing to the religion. The point is the closer she gets to the target, the closer she'll get to the targets friends, the more chances there are for her to talk about her religion. This then increases the chances of someone being interested enough to go in for a 'friendly chat' even if it's only because they're not confident enough to say no.
They use these techniques because they work. Not all the time, and not a lot. But it costs them nothing so why wouldn't they? It's marketing, plain and simple.
Sorry, I'm getting off topic. I just find things like religious recruiting quite fascinating.
I also don't think it's very kind of you to say that people who are involved in cults are not smart. Some of them are very intelligent, but are going through difficult times, some are raised by family members already in that cult and know no other life. Some people are just easily influenced or feel a little lost. Cults (in fact most religions) are know for their skill in identifying those who are most easily manipulated.