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**  
![enter image description here][1] ![enter image description here][2]

As to *why* they keep their weapons drawn (and their hands inside the circle), the [Star Trek: Deep Space Nine - Technical Manual][3] 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.*

  [1]: https://i.sstatic.net/sN6sV.gif
  [2]: https://i.sstatic.net/y9hJs.gif
  [3]: http://en.memory-alpha.org/wiki/Star_Trek:_Deep_Space_Nine_Technical_Manual
  [4]: https://i.sstatic.net/0iOfg.png