Not only should they NOT have a threaded end on to, prevent filling of fresh water tanks, it also helps speed up the selfish idiots who think that taking ten to fifteen minutes of slow ritual, while dumping, is just fine. Even though there is a line backing up behind them. I can only imagine how bad it would be if the same type of clods add a tank flushing ritual to their list of OCD procedures while dumping. It's simple, put a pair of disposable gloves on, connect the hose, dump the black, dump the grey, put the hose away, move on. If it takes more than five minutes, and there is somebody behind you, you need to rethink the ritual.
Unless you are putting the RV away for the season, there is zero need for a tank flush. Once you dump, use the onboard water to add a 2-3 gallons to the black tank, as a bonus, toss in a small amount of liquid laundry detergent, and drive away. After decades of doing this, and dumping many hundreds of times, I have never seen a need for a flush system, or had a tank clog, while on the road. Sitting for months at a time, in a seasonal site, yes, on the road, never.
