Keeping all the “crap” people lose in the park for a long time might be unreasonable, but keeping something for an hour, or a day, is certainly not. You seem strangely annoyed that the OP was disappointed that her reusable child’s cup was tossed in the garbage immediately after someone sent it to lost and found.
If I dropped my hydroflask water bottle in the park I would be pretty upset if the lost and found CM’s immediately threw it in the trash when someone turned it in. Sure I can survive. But it’s still surprising and frustrating.