Association documents outline what is owned. In many condominium associations the “owner” only owns the interior of their unit through the first two coats of paint. They do not own into the walls - even inside their own unit. It is common area. The door to the unit and the walls in the hallway are also common area and not owned by the individual.
What is individually owned and what is common area is explicitly spelled out in the association documents and known to all at time of purchase.
Why does the association board fuss over it? Because people push the envelope and it sets a precedent. If I want to hang something ridiculous, e.g., a porno picture on my door I can point to the mezuzah as a precedent. Then people will post pictures of Elvis or Jesus on their doors. You get the picture.
Some people are not cut out for condominium living. They want to expand their space and decorate the hallway outside their unit.
So in Jewish retirement condo areas, is there a bylaw allowing a small mezuzah, so no one has that problem?