The issue is that the policies are vague enough for immigration to enforce stricter entry and extension rules. There are lots of points they can play with: 500k, ongoing activity/work, certified activity, etc.
Non-B holder speaking... The DTV rules and conditions are as fixed as old visa conditions and requirements, which can change at any time. It is not unusual to be asked for an additional document compared to the previous year, and there is no set timetable for these changes. Keep in mind that the people granting your DTV are not the ones responsible for allowing your entry or extending your stay in the country.