I am not disagreeing that Safari's default behavior is stupid (open "safe" files). The fact the the install process is benign is not "fortunate" though -- to q

I am not disagreeing that Safari's default behavior is stupid (open "safe" files). The fact the the install process is benign is not "fortunate" though -- to qualify as "safe" they're running Apple's installer from a pkg. If they provided their own installer, the installer wouldn't be considered by Safari to be "safe" and the whole thing wouldn't work.