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).