I suspect that when you buy decaf beans from a roaster they don't aren't the ones decaffeinating the coffee beans. I think roasters have a lot less choice when buying decaffeinated beans so I bet your theory is correct.
The decaf I buy states they have close ties with the farmers. The company they use for decaffeination seems to be a service-oriented company. You send them the raw coffee, they decaffeinate it, and then send it back with recommendations for roasting.
So, the "roaster" (if they don't use a separate company for that) does have the ability to select the exact same beans between their decaf and regular coffee products. Whether they sort the beans they get from the farmers and choose which go to be decaffeinated according to some criteria, is a separate thing.
I've noticed many roasters have only one decaf option. But there are exceptions. Equator Coffees has many blends available in both regular and decaf, with the decaf costing slightly more (e.g. $15.30 vs. $15.75 for a subscribe-and-save 12 oz bag). They advertise "decaffeinated using the mountain water process of caffeine extraction", which I believe is the same as the article's Swiss water process. Not sure if they have their own equipment or if they assemble their blends, send them off, get them back and roast them.
