As a registered nurse I think of it in terms of my job. Those nice girls at a doctors office are NOT nurses (for the most part), at best their medical assistants with some training. They know there basic skills (though many don't take blood pressures correctly IMO) but they don't know why they do the things they do. There is theory and research behind everything in the medical field, things you learn with a formal education. Just because you can perform the manual task doesn't mean you understand why your doing it or what your looking for. Would I want them doing much more than drawing blood from me? No.

I assume if a VT performs a spay (completely negligent and illegal on the vets part) they know the procedure but do they know all the rules of sterile technique (trust me there is quite a few) and the intricasies of anesthesia?
If a VT perscribes meds do they know about interactions with other meds, labs that need to be monitored, even how the drugs themselves work? I highly doubt it. These are things veterinarians go to school for 7+ years to learn not something on the job training is going to impart.

If I were going to a vet the allowed a tech to perscribe or perform surgeries I'd be finding a new vet and writing a letter to their state board.