Even though I don't build engines for a living, I can understand the machine shop owners point.
I don't like when my customers watch me put lettering on their cars, because they always seem to want to have a conversation and chit-chat with you while you work...I can't concentrate when they do that, and sometimes I goof up.
Liability is a big concern when they're in the shop and want to see how our machines work...

Just my 2 cents...