You need to eat calcium so that it can bind with oxalate in the stomach and intestines before it moves to the kidneys, the risk is if unbound oxalates reach the kidneys. I know it sounds counter-effective, i thought so at first too.

Here is some medical paper about it with some other methods of preventing the stones :https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1455427/

hope you find it useful :)