We'll have to agree to differ @WhatsTheUseOfWorrying - because there are many, many idiots who insist on being able to touch any dog they like. And a lot of children don't just "approach" dogs - they charge up to them, often shrieking and waving their arms.
Every dog should have the right to defend itself. Usually it doesn't take anything more than a grumble to put people off - though there are some than then complain that it is a "nasty" dog. No, it's not - it's saying "I am ill/ unhappy/ frightened/ blind/ old and don't want people I don't know mucking me about". That is what a growl is for.
I will also add that very few dogs actually intentionally bite. Most will warn with a growl, and will try to get out of the way. If someone insists on being "in their face", they will air snap. If they do get forced into a situation where they feel they have no option, they snap at hands.
If a dog goes for the face it is because it actually is a bad/aggressive dog, or because the face is too close to the dog's - eg a toddler face (and the adults there should have kept dog and baby apart), or one of these twats who thinks it's very clever to approach a dog, walking stiffly and growling at it until face-to-face to "dominate" it. (IMO any adults doing this deserve what they get, but I wouldn't want my dog put down because of them, so I wouldn't let this happen.). The dogs also retreat once they have made their "aggressor" (for that's what they see them as) back off.
Cesar Milan has a lot of stupid, threatening, needlessly provocative behaviour to answer for, and people copying his techniques are often asking to get bitten IMO.
Edited to add: This particular type of dog is dangerous because they don't give any warning or an attack - and when they do attack, it IS an attack, not defensive behaviour. And because it is an attack they go for the vulnerable parts of the body - they'll bite at legs or use their weight to bring the victim down, and will often then turn the attention to the head and neck. They aren't attempting to escape a threat- they are prey-driven.