Why is the rental agency im renting my house from trying to charge $150 extra on rent for PETS?

i have a cat that i love dearly. A big fluffy clean persian cat to be exact, and the rental agency charges $150 extra a month for rent for pets. now i mind you i have a clean house and my cat is clean and well trained to go to its litter box. So why would a cat be a problem? she isnt even an outdoor cat. Now i could probably understand a dog because it seems as if the previous tenants kept a dog and i can see where there were holes dug up in the back yard and old poop off in the corner of the back yard. I just cant see where a cat can be any kind of threat. i need help on this because im tired of hiding my cat everytime the landlord comes around.