If I may be the voice of reason here.
Photogasm, if you weren't killed you've lost essentially nothing. You cannot have anything irreplaceable within two weeks of playing. And rebuilding, re-crafting and all your possessions will simply improve your character. During this game you will constantly and to no end be replacing your items with better quality ones anyway. So what they took is only a minor inconvenience in a long and endless road of cyclical upgrading.
And 99% of the time you won't want to stay where you first spawn and will move to an area with better quality resources or into a village with like-minded folks.
So my suggestion, IF you enjoyed the game whilst playing, is to ignore the Why and focus on the How.
To directly answer your question as to "Why they did this to you?" the answer is because they can and you were a target of opportunity.