The gun rights supporters oppose training requirements for the same reason pro-choice supporters oppose any forms of restriction on abortion.
This being the second reply apparently presuming that I was referring to some sort of government-approved licensure process, I feel compelled to point out that I by no means meant to imply that either situation should require such approval.
I was merely pointing out the absolute fact that properly trained people are far less likely to misuse a tool than people who are not properly trained.
The government cannot ban X, but they can require X is only available after filling in form 3940-subsection-C in triplicate and submitting to a federal agency which has an annual budget of $50 and a two-year backlog on processing the paperwork.
Proof in the pudding: the Marihuana Tax Act of 1937