I have another reason for not liking the sitter to touch the cards, or at least shuffle them.
I don't do reversals. When I've done readings for others, when giving them the deck to shuffle, I would tell them, "be very careful not to reverse any of the cards". I would show them where I had marked the deck at one end, so they could keep the ends all pointed the same way.
Invariably, I would end up with a spread with half the cards reversed.
OK, it's a minor annoyance to have to sit there and turn the cards upright, then go through the deck after the reading and get all my cards pointed the same way, but people who can't follow very simple instructions accompanied by visual aids really get on my nerves. And that seems to be about 99% of the population of Earth.
If I get back into doing readings for others, I'd be inclined to fan them out and let the sitter pick the cards for the reading.
So I figure I'm best off shuffling the cards myself to save the aggravation.