If I am reading the announcement correctly, JetBlue has restricted free standby to the flight immediately prior to the pax's ticketed flight:
Originally Posted by JetBlue.com
For travel to the same destination, customers may change their reservation to standby travel only on the flight immediately prior to their original departure without an additional charge. Standby travel is not available in markets that only have one flight a day or if there are no flights prior to the original departure.
Previously free standly included any flight on the same day, and also the last flight the previous day for pax ticketed on the first flight of the day.