Honestly burning them, in some ways, is a service to the community.
It is faster than demolition, which takes, on average, a decade or more to get done.
It is cheap, although there is obviously cost to the fire department.
Apparently it is -- at least so far -- safe as nobody was killed in any of these fires. Apparently the arsonist is careful to make sure that the properties are vacant when set ablaze. None of the firefighters have been reported to be injured either.