I just read this on CNN: Cemetery workers accused of reselling plots, and apparently it is a huge thing. But from my standpoint it is pretty normal to do just that. In Germany you have to rent your grave and pay for it for 25 years. If the grave isn't rerented and paid for, the grave will be bulldozed and get a new tenant.. There is not enough space to have a final resting place forever, and once the body has gone dust to dust there is no reason to reserve that spot, unless the family explicitly does and pays for another 25 years. I'm not sure how the rule is for mausoleums and kryptas, if they are privately owned things might be different.
It is a luxury to have enough space to have enough graves for everybody.. that sounded better in my head. Regions that have been densly populated for a long long time never really had the option. I find it completely normal to reuse used plots when the person in there has been dead for a long long time and no family is around to remember.