This is similar to a rear-end car crash. The driver who smashes into the rear-end of a preceding car is almost always to blame - regardless of whether that preceding car was within the lane markings or even braked too hard. YOU as the following vehicle are required to allow ample spacing to avoid a crash.
In this incident, it might also be helpful to take the scenario a bit further...let's say the already-parked helicopter was much further out of the box...is it his/her fault still...or the landing pilot? What is the parked helicopter was half-way over the other parking spot...is it the fault of the landing pilot yet?
You don't land an aircraft unless you've ensured it is safe to do so. You can blame the umpire all you like, but the fault (and root cause) lies with you. Own your mistakes. I own a lovely acreage of mistakes, which I carefully tend to and nurture, so I NEVER forget them.