it's kind of like driving your car unannounced onto someone else property, just that usually folk tend to be more aggrevated by a helicopter due to the profile of the things.
Technically you are liable for any damage to property you may cause.
They can also kick up a fuss if they considered what you did dangerous, then it is up to the Cash And Agro to prosicute if they have a case to prove.
The landowners permission is really to try and make sure there are no hassles when you land, then in the event of a mis-hap, you had permission to be there, which will support any case you have.
Remember, a polite apologetic pilot will get further than rude objective one!