Good answer Robert. While it can be frustrating to take your animal to a non-experienced vet, the expertise available should not be underrated. The fact that "you could probably do it just as good" shows your misunderstanding of what exactly is involved when prescribing treatment. It is these types of passive misunderstandings that show the importance of FDA control over drugs, and the reason why prescriptions are necessary.
If you are unsure of what exactly is involved in your mistake, then for your interest, consider talking to your veterinarian about what is involved in writing a prescription. In addition to finding the problem, and deciding on the appropriate drug, a veterinarian must also consider how this drug will enter the body, if it will travel to the appropriate part of the body to be effective, how it will be metabolized, and finally how it will be excreted. To make things all that much more complicated, the veterinarian must predict any outside factors, prexisting symptoms and other ill effects that may complicate any of the above decisions.
I like to use the antibiotic Amikacin as an example. If an infection is found to be susceptible to amikacin via culture and sensitivity tests, an accurate dose based on weight and drug concentration is calculated. This part is simple. Next the veterinarian must decide how to give this drug. Will it be orally, or by injection. If the infection has been walled off by the body, will the drug be able to penetrate it? Next is to consider how the drug will be metabolized. Is there any reason to think that this reptile's liver is compromised? Should bloodwork be done to confirm this? Amikacin is excreted via the kidney. It is toxic to this tissue, and will kill the it. This happens if the kidney is not functioning 100%. If even slightly dehydrated, the kidney slows, and is susceptible to deterioration and/or failure.
Now there are hundreds of drugs used in veterinary medicine and about 6500 species of reptiles. The ability to research, discuss with colleuges, properly use reference materials, and extrapolate from personal experience and education all allows a veterinarian to make these decisions better than even the above average joe.