Wow - OP is getting some stick here for their first post.
What happened to welcoming new forumees?
What the OP likely doesnt realise is there was a spate of false reports to the NHTSA about suspension failures some time back on the Model S, hence a lot of people here are a bit twitchy on the subject.
This is the only report of suspension damage on the MX I have seen on here (other than major accident of course) therefore there is no pattern of failure in normal use - as others have said in various ways,
I think you should just count yourself unlucky you must have connected with the post in such as way that excesive force way placed on the arm and it broke. Any sign of accident damage and quite understandably no manufacturer will take responsibility as no way can they possible know what forces have been involved.
Just a bit unfortunate, but hey get it fixed, move on and enjoy the car.