Never thought of that.  We had a mandated financial report at work that
was needed Right Now.  It was a yearly report.  The software had a bug
in it, and the numbers simply didn't add up.  It was one page.  The
accounting folks knew what the numbers were supposed to be.  I suggested
to the Little Cheese that I just edit the print file and fix it.  I told
her it would take two hours, a few seconds to do the edit and the rest
to figure out how to make it editable and then convert it back to a
print file.  The Big Cheese came by and asked what the problem was.  He
said my solution was ridiculous.  Just import into Excel, or retype it.  
Å clerical person could do that, you didn't need a computer programmer.

If you agree that everyone should get an initial refraction done
professionally, how are your going to enforce only allowing someone to only
make an minor adjustment to the initial Rx? Either all refractions have be
done by a licensed OD/MD, or none of them would have to be.

One could argue about how long a refraction should be valid for before it
expires, and in some states a Rx for contacts expires one year after the
exam date, and in other states it is two years or longer. But expiration of
a professionally done refraction is different than coming up with a new Rx
on your own.

But (if you really, really want to) there is always Photoshop.

I can see why prescriptions are required for drugs, because those can
be easily abused or sold in the black market. But to me, eye
prescriptions don't seem that critical.

My analogy is shoes. You could probably mess up your
feet/knees/legs/back by wearing the wrong size or type of shoes, or
improper inserts long term. Yet a shoe prescription from a podiatrist
is not required for people to buy shoes.

Of course, everyone should get an initial refraction done
professionally. But what's wrong with buying disposable contacts that
are a little higher or lower than your prescription to see how they
perform? You're not going to be seriously injured. Maybe get some
headaches or eyestrain, that's all. And if you do have problems, then
go see the optometrist (just like you'd see a podiatrist if shoes were
giving you problems).