Who is responsible for the plumbing expenses?

I live in an apartment complex in Gardena, CA. We've been having problems with the plumbing on a consistent basis. The owners sent us a letter a few days ago stating that the next time we call for a plumber the expense will be added to our month's rent. Is that legal? This is not our home! Are we responsible for the plumbing? Please help!!!! The question is in regards to us not causing the blockage. i.e. Old pipes. The tree roots breaching the pipes. ect., ect.