We're regular holiday let-stayers as a we have loads of kids (5), and so have seen a variety of 'house rules' over the years. We're currently staying in an otherwise lovely holiday let that has a house rule that 'all refuse and recycling must be removed from the property'. We're not staying very remotely (small town on the coast close to other small towns) so I can't see any 'need' for this rule other than to cut down the cleaning at the end. Having to remove all our litter (which I'm trying to keep on top of as we go) is making me increasingly cross. I'm not saying I definitely wouldn't have booked if we'd seen this rule, but I certainly would have considered other options more carefully first!
Cheer me up with your tales of the weirdest - unexpected - house rules you've come across in a holiday let!