star trek - Why does everyone face the same way on the transporter?

When the away team prepares to beam down, they invariably assume the transporter formation: standing upright, usually with arms slack aside, all facing the camera. When they beam down, they reappear in the same formation and poses, as that's how the transporter works.

However, they do this even when it makes little sense, such as when beaming into uncertain, hostile environments (where they usually proceed to draw phasers and look around in a defensive circle. Sometimes it gets more silly - on a few occasions, they prep their phasers, holster them, beam down, and immediately draw phasers again.

We know that the transporter works with any formation or pose; people and objects have been beamed lying down, holding large objects, holding each other in emergencies. But routine transportation always involves everyone standing facing the same direction.

Why do they do this? Is it somehow related to how the transporter works? Or is it purely for out-of-universe reasons?

Answer

The short answer is that it seems to be standard policy for Starfleet personnel to transport with both hands free, facing the front of the transporter. There are a few instances in Enterprise, Voyager and TNG where the crew beam into hostile situations with their phasers drawn or standing in a circle (or both) but these are very much the exception rather than the rule.

                      TNG : Legacy                                             TOS: Day of the Dove

As to why they keep their weapons drawn (and their hands inside the circle), the Star Trek: Deep Space Nine - Technical Manual seems to suggest that it's a question of transport efficiency for a person to keep their whole self directly above the transport pad.

... transport platform. Performance is somewhat degraded if the unit must target the subject off-platform, especially in widely separated areas of the station. The most efficient transports occur between platforms of like design, and even between platforms of dissimilar design, as in a beam-out from the ops platform to one aboard the Defiant. Since all transports involving living entities are zero fault-tolerant, degraded system performance is related only to a decreased amount of mass delivered per unit time. Transports employing lower resolution scans of nonbiologics may tolerate nanometer-scale voids and 0.001 percent molecular recombination errors.