Do you have access to Verizon's books? If not, then you cannot definitively say that Verizon didn't gain any new subscribers out of the iPhone deal. Nor can you definitively say whether or not it was a good business decision.
Let's say your assumptions are right - why should their decision be baffling? A revenue increase of $30+/line for everyone who switched to the iPhone isn't chump change. That revenue increase will more than pay for the subsidies over the term of the contract. They also prevent those users from jumping ship to AT&T. It would've been a terrible business decision not to carry the iPhone.
It was a win-win for both apple and verizon.nApple gained sales from those who were not willing to jump ship; Verizon gained revenue from those who switched, and they gained subscribers (people who jumped ship from AT&T to Verizon.). Many people did not want to sacrifice their coverage and quality of service to get the iPhone - I think during the launch they mentioned that Apple was constantly flooded with calls that were something to the effect of "When is Verizon getting the iPhone? I'd buy one immediately if they get it". And until recently, AT&Ts 3G coverage was pathetic compared to Verizon's. I'd much rather be on EVDO than GPRS/EDGE. I had no 3G where I live until a little after the iPhone 4 launched.
There's been so many numbers published. In aggregate, there wasn't any massive switch anywhere when the iPhone became available on another carrier, except for originally when AT&T picked up some subs, but Verizon still held their own without the iPhone.
It sort of makes sense that Verizon did what they did, what I don't get is why people are buying the iPhone, as it is an antiquated device on Verizon without 4G LTE. However, without the iPhone, I think Verizon could have up-sold almost as many users into DROIDs, and retained their customers in a similar manner.
True, if you live on the edge (no pun intended) of AT&T 3G, or totally away from it, you'd want to have Verizon EVDO. But that doesn't really impact anything, as those users would just be EVDO Android users without the iPhone on Verizon.