Is there any reason you think they may be?
No, not really.
They cannot really be said to be publishing it. It is not an act of their choosing.
It would be different with a newspaper that had chosen to carry an article.
Unless they were responsible for the graffiti they cannot answer for third parties.
On that basis the councils would always be liable for any graffiti.
Sorry if that is bad news.
Can I clarify anything for you?
The council don't have an obligation to clean it up on private property and, in fact, no rights to do so.
There are offences in displaying something that would offend against the public order act. However, they would have to show that the house holder was responsible for the graffiti.