It's common for the reservoir host of a zoonotic virus to be tolerant of it. MERS coronavirus appears to cause mild or no disease in dromedary camels ( source ), but kills about 35% of confirmed infected humans. ( CDC ) Sin Nombre hantavirus seems to be mild in the deer mice that spread it, despite ~36% fatality rate in humans. ( source ) Mosquitoes are efficient vectors for flaviviruses like dengue and zika in part because they have adaptations we lack that protect them from the virus. ( source ) Also, human communities are host to several viruses: about 90% of people have a herpesvirus infection ( source ) with similar numbers for polyomaviruses. ( source ) Very few of these infected individuals show symptoms.
The wide prevalence of these asymptomatic infections shows that the virus is successful when it can replicate while the host remains healthy. In general, virus reproduction kills cells, and when cells die faster than the host can replace them, this causes symptoms up to and including death. The host's immune system supresses virus activity, and a virus that can't evolve ways to avoid immune supression will be wiped out. But if the virus gets so good at avoiding the immune system, it will kill the host, which is bad for the virus. So both host and virus tend to evolve to a point where the immune system wins most, but not all of the time. There is a lot more to it, but I'll stop here.
In conclusion, we don't really need to look for special properties of bats to explain their tolerance of coronaviruses, even though, as iayork points out, there are reasons to expect bats might be more resistant.