One of the things I found most interesting about ala kachuu is that the women in the kidnapping family are often the ones who end up forcing the kidnapped girl into the marriage. This was especially apparent in the documentary; it was not comfortable to watch the women- the ones who should have understood the difficult position that the girl was in- push her the hardest to accept the marriage.
It's also important to remember that ala kachuu is not the same in every case. For some couples, it's more like eloping and planned by both. In fact, some of the research suggests that the more traditional form of ala kachuu was much more consensual. But most kidnappings are not consensual now even though they usually result in a marriage. Most girls (and their parents) feel that they have to accept the marriage because of the shame of being kidnapped. She is not necessarily raped; simply being in another man's house overnight is enough to shame her, even though she has no control over what has happened.