I've attached a link to the FCC rules about this. You need to scroll down to the FAQ, where they cover the "common antenna" issue.

Basically, it seems that your landlord can require you to use the dishes he has on the roof, if a) the cost is no greater than the cost would be for you to have your own dish and b) the service is the one you want and c) the signal you get is the same quality as what you would get if you had your own dish.

So your landlord can't charge you more per month than you would pay if you had your own dish. The landlord has to provide the service you want--if you want DirectTV and he only has Dish, then you can get your own antenna. And the picture quality you get has to be as good as if you had your own dish.

Here is a link that might be useful: FCC rules