The yahoo.finance tables are not official Yahoo YQL tables. They were created by 3rd parties or were intended as examples. They are also rate limited by the finance site or are occasionally blocked. Also, Yahoo finance data is not approved for commercial usage via YQL.
The YQL Service can be used for commercial purposes without prior approval from Yahoo. Developers using the YQL Service fetching data from a content provider need to conform to the Terms of Service of the content provider. Please note that data available on the Yahoo! network (e.g., Finance, Weather) is not approved for commercial usage via YQL, unless mentioned otherwise in the Terms of Service.