So when you go to brush your teeth you don't wet the toothpaste and toothbrush from the tap? Is it the fear of tap water that centers around this? I am curious about this as it seems the answers carry some cultural significance.
No - toothpaste is in part an abrasive. If it's wetter, the abrasive is less effective at removing biofilm/plaque, and the chemical components are diluted in situ. Your mouth is plenty wet enough as-is.
That implies toothpaste isn't designed with this water dilution in mind, as that is common practice. If I don't wet it beforehand it's too thick and doesn't get distributed as easily.