Three reasons:
1. Pets do cause additional expenses. Labor, wear and tear, rule enforcement etc.
2. Management tool, pet fee keeps away people with multiple dogs
3. Revenue center, like any business parks are in it to make money.
It is a no win scenario. We don't have pet fees, so people complain about our limits on the number of pets, our rules regarding tethering, unattended pets, kenneling, our rules regarding where to walk them, and rules about picking up after them. If we had fees they would complain about that.