A couple of points I need to make:
1.  Around here it isn't the mfg. who makes it prescription.   The
government dicataes that it is prescription.  
2.  Precriptions don't dictate controlling of the market.   You can
have an item be available only by prescription that has a dozen
generics to compete.   (Thryroid pills are a good example).  

BTW, I use K/D prescription food, around here I can
it alot cheaper from my vet than I could get an equivalant
amount of IAMS at the pet store.  (Maybe because the
vet gives it to me at cost while the pet store maks the
IAMS up more than 100%?)

No corporation would choose to have an
item be prescription rather than over the counter.
Escpecially now when regulators are strongly
considering preventing making ads for prescription
items illegal.  

But around here the government dicatates that it be by prescription
only.   For Hills there is no choice.

Around there goverment dicatates it can't be sold openly.  

Why would Hills want it to be by prescription when the prescription
process would cut their market share dramatically?  Around here
a doctor can choose not to prescribe it, to go other routes
to trea the problem.  

For Hills that makes marketing much more difficult, they have to
encourage doctors to prescribe it which around here means
they have to send reps to visit the doctor.  It would be much
easier to sell to the general public in a public store.

Also around here the mfg. has a lot more hoops to jump through
to get approval than if it was available to anyone.  

Did you follow what they had to do to get Erbutux to market
as a presription item before they finally got approval?
And that was for a drug that costs $12,000 for a one month
supply.  (Yeah....that Erbutux......the one Martha Stewart went
to jail over.