WRGB operated its analog signal on channel 6. This channel is right below the bottom of the FM radio band. Most FM receivers could get WRGB's audio on 87.75MHz on the FM dial. Indeed, all channel 6 analog TV stations could be received at 87.75 on FM radios.
When WRGB (and the other analog TV stations) switched to digital in June, the FM audio was supposed to go away. Much to the surprise of many observers, in the Albany/Schenectady area, it didn't! WRGB continued to operate their analog audio transmitter, on the same channel 6 as their digital transmitter. The station even petitioned for waiver of the normal power limit for channel 6 digital stations in New York State, complaining of poor digital reception.
The FCC has been bending over backward to accommodate the DTV conversion. I think most of us assumed WRGB had Special Temporary Authority from the FCC to leave the audio transmitter on the air, as some kind of experiment to see if interference would result.
Apparently we were wrong; the analog audio transmitter was not authorized. It's hard to imagine an admission more to-the-point than:
"We do not have FCC authorization to transmit an analog signal. We only have authorization for a digital signal at this time."
(VP and GM Robert Furlong, on the WRGB website)
It should be noted two LPTV stations have done the same thing - WNYZ in New York City, and WLFM Chicago. Many full-power rules, however, don't apply to LPTVs; these two operations may be perfectly legal.