i like the theory.
it emphasizes how important and beneficial live foods and 'natural' protein consumption can be. I think the digestive system parasites may be unable to support the change in their environment considering all the snail matter they are now exposed to. Consider also the water chemistry differences existing in a planted tank in comparison to the transport conditions and previous shipping conditions.
i recon there are unknown benefits (trace compounds) to living in a planted tank in the first place anyway.