People like to touch things. Go to any car museum, show or display and despite all the Do Not Touch signs displayed in full view, people will touch the cars.
I used to go to a lot of auto shows and it always amazed me. The same goes for museums. If guards and crowd ropes aren't present, people will go right up to a centuries old painting and touch it, knowing they shouldn't.
Ignorance abounds and it's nothing new to anyone here so the onus is on the host to let it be known that no one touches their system. Don't let that little voice in the back of your head tell you you'll be considered weird or eccentric and hope against hope that your guest will behave. They won't. And, you know it, so some proactive measures have to be taken on your part.
I had a friend who'd intentionally touch, tweak and manhandle things just to inflict damage. He was jealous of anyone who had it better than him. It could be anything, a book or magazine, some stereo gear, or even a car and he'd mess with it. It taught me that there are myriad reasons why people do stupid things and not to trust them. Don't even try to figure them out.
All the best,