Many people value their dogs because they believe their dogs will protect them. Protect them, in part, from the perceived threat that is posed by crazy people like you who run. Many people like being out in public with an unleashed dog, as its similar to being out with a gun.
Instead of having to pull out their gun when they feel threatened, they move around with an unleashed dog. The dog scares many people away.
A dog charging at you is like them pointing a gun at you, or even firing shots in your direction. This threat to your health, safety and sanity is not interpreted the same as if they pulled out a gun and aimed/shot it at you. Conveniently the consequences of having their dog threaten you are far less than if they threatened you with a gun. It is somehow more socially acceptable to be out in public with an unleashed dog than to be out there with a gun.
They get mad with you for being upset because:
1) their dog threat did not work properly, since instead of running away from them in fear you are confronting them (confront them and you become a bigger threat)
2) you are questioning their basic judgement (and their irrational fear)
3) they know that ultimately they have less control over their dog than they would over a gun trigger
4) if you overcome their dog, or the dog misfires, they may have to deal with you without protection
The owners almost always say "he won't bite". They do this in an attempt to absolve themselves of some responsibility ("I did not know the gun was loaded")
Some might say dogs are less likely to kill you than guns. I have been threatened by both, and the fear was equally real