Great question.
The obvious answer is "of course there should be formal training. No one should be allowed near a firearm without lots of formal training".
This is wrong. If you take out the word 'formal' then it's closer.
How did people get 'trained' before there were laws requiring formal training? How did people get 'trained' on other, more dangerous tools?
Someone passed the knowledge down to them. The training was part of their upbringing like every other learned skill. I'm sure some of the training was 'bad' or less safe than others, just like parenting skills are not equal. This is where a better means of communication is helpful. On the internets we have access to loads of information, some of it down right dangerous, but all the same, easy access to information. This is helpful to 'training'.
People should be 'trained' to handle tools that can kill, but I don't propose a government mandated 8 hour class to buy a chop saw, or a nail gun. The training to use such tools is not hard and consists of a few simple rules, it can be learned by almost anyone.
The free market is filling a void for people that don't have access to informal training in the form of 'schools' that, for a price, will teach people various skillsets with firearms. If the government wanted to do something, they could have low-cost or free training centers for basic skills with firearms, yes, even for kids. Crap, what am I saying, this would never happen, or if it did, it would end up in bureaucratic red tape and lawsuits. ;->
"Shall not be infringed". Even if it means there are accidents and deaths.