didjfan,
I can only guess at the rationale of the USFS in declaring the site "day use only," but my guess would be that the biggest concern is toilet practices. In the day time, hot springs users might tend to do their business farther away from the stream for privacy, but under the cover of darkness, many people use the area right behind the bushes by the grassy area. I have walked back there and it is gross, plus it smells. That's just one guess, though, and it is probably more than one reason. Others might be increased vandalism at night, more "partying," more trash, greater probability of campfires, the fact that rescue would be more difficult, or even that pesky candle wax that keeps on collecting up from night users, even though some volunteers keep cleaning it off as best they can.
I'm not sure what this has to do with them leaving their desks and doing some work, though, or why BLM rules might be different. I can see the enjoyment in night soaking, and I do believe that most hot springs users, day or night, are careful not to harm the area and to remove all of their trash.