Because some people are not as savvy or frequent of travelers as others and would think that something like this incident could happen. Also it notes that the urn was heavy perhaps that in itself was reason enough.

That said I think the one missing element here is that it does not appear that the PAX marked the urn as containing human remains. If it was that would have most certainly have been stated in the suit. That is probably why the TSA opened it - could not see through thus opened it to inspect. What happened after that is anybody's guess. TSA certainly has a responsibility in this case but so does the PAX if it indeed was not properly marked.