Yes, this is a common issue, and agree that it can be a safety issue too and LR could be handling it differently.
There is a pivot bolt in the d-pillar where the rear hatch struts attach. It has threads, and screws into the d-pillar. This bolt can back out over time, causing the right side strut to become disconnected.
I had this happen post-warranty about 6 months ago. The pivot bolt was damaged as a result. I was able to replace it (for under $2.00) and it took less than 5 minutes to repair. Not sure why your dealer charged so much, unless the nut got stripped (the part inside the d-pillar) which could be difficult to replace, or perhaps the strut got damaged.
It would be a good idea to inspect and tighten these on a regular basis as a precaution.
Here is a previous thread on the same topic
http://www.landroverworld.org/forums/showthread.php?t=15050&highlight=pivot
Make / Models : Model/Build Years:
LAND ROVER / LR3 2005-2006
Service Bulletin Number : 501016
NHTSA Item Number : 10019761
Summary Description :
TAILGATE SUPPORT MOUNTING. RATTLE FROM THE UPPER TAILGATE AREA. *TT