My parents have a similar problem, but with their own home. They like to spend the summer months touring the country in their RV, which leaves their gigantic house unoccupied during those times. Lately, they've been loaning their primary home to various relatives who want to come here for vacations. While their home isn't exactly in prime tourist location, it's huge (5 bedroom, 3 bath, indoor and outdoor kitchen, 30X60 foot pool on a deck nearly the same size as the footprint of the rest of the house, several acres of land), it is centrally located to many South Florida destinations (Upper Keys, Everglades, Miami Beach, Downtown, Coconut Grove are all within an hour of the house unless traffic is horrifically bad). They just ask that their home is left in the same condition it was found in. Sadly, this hasn't always happened.
I think the posters who've guessed about the attitude that free=we don't have to take care of the place, are right on target.