Yeah, it’s for ease of machining, nothing else. It’s extra work to (a) cut a lockring thread or (b) part the excess threaded area from the hub, which would then require a different spacer / bearing setup on that side.
The LBS are idiots. If you can use a fixed sprocket with its thread width, then a free wheel will work with no issues either. The forward motion stresses are the same.
I asked the Sheldon the same question about some C Record Hi-Flange track hubs that I wanted to use a freewheel on. From memory he said in theory it wasn’t advised but in actuality it didn’t matter much and I did run a 5 speed freewheel on the track thread in a wheel I built for a special ride/show bike.
I think most single speed freewheels will be fine. As long as it winds on enough … use your brain, if its too wide on the back end and doesn’t thread on far enough then try another freewheel.
That’s the key key point. Definately get fixed/fixed. I have a fixed/free hub which shits me all the time because I can’t put two fixed cogs + lockrings on.
Oh, if the opinion of these guys isn’t enough, Sheldon agrees:
There are double-fixed flip-flop hubs, and, to me, this is the most desirable configuration. This arrangement is the most versatile, because you can set it up either with 1 or 2 fixed sprockets, or 1 or 2 freewheels.
Any standard track hub can also be used with a single-speed freewheel just by leaving the lockring off. The thread is the same. Sometimes people worry because the hub thread isn’t as deep as a freewheel specific hub, but this is never a problem with a single-speed freewheel.