Why does PhRMA oppose the public plan?

The CEO of Lilly gave a speech today opposing the public health insurance option.  Now, a knee-jerk liberal like myself says "Yes, that's just big business quashing reform once again."

But when you stop and think about it, why are they so afraid?  A public plan would mean health insurance for 47 million more people.  That's an extra 60+ million doctor visits every year.  And you know that at least half of those visits would end with a prescription being written --- probably more like 3/4, if you're as loose with the scrip pad as I am.  

So a public plan should be a huge boon to PhRMA.  Why are they so afraid?

I have my thoughts, but I'm interested in what others think.