So if someone issues a death threat, etc. you call the police, they get a subpoena, and track down the idiot who posted it.
Once this happens a few times (in every locality where Yik Yak is used) word gets out not to do that. Or, Yik Yak and law enforcement are overwhelmed with subpoena processing for trolls. If Yik Yak can't process requests fast enough is there some sort of penalty they face? If Yik Yak corporate headquarters are out of state or out of the US, how would such penalties be imposed?
Or, signal to noise ratio gets way too low and the app collapses under the weight of trolls. Or the trolls enjoy trolling each other on it (4chan).