One thing not mentioned in the abstract (but perhaps it's mentioned in the paper) is how many missionaries do door-to-door sales when the return home. Many don't, but a nontrivial number do. I suspect that that missionaries play a role in the popularity of MLMs: either because of former missionaries joining MLMs (because some missionary skillsets are applicable to MLM marketing/selling), or because a nontrivial number of people in Utah have a soft spot for missionaries and I think MLMs often exploit that same soft spot.