The purpose of the tipline is to keep crazy people calling in with tips so they can present them during Casey's trial that there are sightings. Crazy people will keep calling in, just like they keep adding money to her account, just like they will write to Casey in jail/prison.
It only takes one person not believing the forensics and biting on a semi plausable sighting to F the trial up. I don't think that will happen, but I think that is what they are hoping for.
I fought for the denial theory for a long time (until the second document dump), but now they clearly are not behaving like anyone that truly in their hearts believes their granddaughter is alive.