I don't think it seems that contradictory. A drop cap keeps a definite limit on how many items can enter the economy, and at what rate. The items per account active rate stays pretty much the same no matter how many idle accounts there are. The only difference is who those items go to.
Whereas with unlimited idling, you get the same problems we had when item drops were first added. Some people will idle constantly in the hopes of getting good hats, new items, etc. And now that we have trading, they could idle multiple accounts constantly. I'd be willing to bet in that type of environment, new items like the Fast Learner would mostly enter the market from sources like that, and as such prices would be set by people who literally had no use for metal whatsoever as they effectively have an infinite supply.
The economy would be doomed for anyone unwilling to idle 24/7. As it is now, idling just gives you an extra 8-12 items per week per account. People have to have giant idle farms to do more than acquire decent amounts of metal.