I don't see why you're so surprised. While it might seem counter-intuitive, this process actually drives more revenue to United, or at least guarantees more of it. Knowing that I can refare for free to recoup fare drops means that I'm willing to book flights a lot earlier than I otherwise would, which not only guarantees my ticket with United but also gives them my money that much earlier.

This refare policy is intended to remove any perceived penalty for early booking and therefore to encourage booking as early as possible. By doing so, it also potentially reduces the loss of passengers who might have waited for a fare drop, didn't see one, and then booked on another carrier who has lower last-minute fares. Moreover, United doesn't really lose anything by this, because refaring to a lower fare simply eats up a lower fare that United planned to sell anyway. Additionally, most refares are issued as vouchers, which have some (non-negligible) breakage, so UA "loses" even less this way... not to mention that vouchers encourage future business, since they are usably only on United. Finally, most people don't even know about refares, so this affects only a small portion of the population.

Overall, I'd say the customer goodwill generated by UA's liberal refare policy, and the additional future business encouraged both through goodwill and the issuance of vouchers, far outweighs any potential lost opportunity cost. IMHO.