Because they want to give us the opportunity to give our minds and underutilized mouse fingers the much needed exercise that they cry out for and deserve while learning to take advantage of other features that Yahoo! has to offer. Go to Historical Prices, hit on Dividends Only and then Get Prices. Vwalla! Not exactly Rocket Science.
Look at the disclosure paragraph at the bottom of the summary page and you will see that Yahoo is nothing but a conduit. They obtain their financial data from several other services, like Thomson, Dow Jones and Capital IQ. Whichever one of those servicer provides the yield info is the one that is to blame. Too bad they don't use Bloomberg. Anyone who has ever used a Bloomberg terminal knows that they are far superior to any of the other providers, but their machines cost a bunch.