It's not entirely true that it is a bull**** practice - it's more of a very out dated practice, one that may have been handed down thru the ages and is now practiced by people who have no idea why it's practiced that way.
It's a literal example of the the following story: A mother who always cuts the end off the ham when baking it is watched by her daughter who asks "why?" The mother replies "that's how my mother did it." The daughter again asks "why did she do it that way?" and the mother replies, "I don't know, ask your grandmother." She does and gets the same set of repsonses and is told to ask her great grandmother. Great grandmother is asked and replies "I had a small baking pan so I had to cut the end off the ham so it would fit".
In that context, way back in the day a few companies sold very short, low profile K valves and when installed on an AL 80 (which has a fairly fat shoulder) there was not enough room for a MK 5 or Mk 10 to be set up with the turret facing down.
Consequently, some shops set up customer regs to face the turret up in order to ensure it would work on any tank the customer may enounter. This was particularly true in shop that rented tanks with those valves or sold tanks with those valves. However, I have not seen more than 1 or 2 of those valves in the last decade so it's a practice that is out dated and no longer serves any purpose, but one that is apparently still applied to Mk 5s Mk 10s, Mk 15s, Mk 20s and Mk 25s by some shops.
The fact is that Scubapro put an additional HP port on the other side of the MK 5 starting in the late 70s, and did so in large part to allow it to be mounted turret up if needed (reflecting the problem encountered with the short valves, as well as allowing the customer the choice of mounting it either way) and that practice continued on the Mk 10, 15, 20 and 25 first stages.
However it's worth noting that the original single HP port Mk 5 had the single port located so that the reg had to have the turret facing down, suggesting that is the orientation Scubapro originally intended.
Which is the long way of saying your shop employs a moron - perhaps a well intended moron, but a moron none the less.