The short answer is that it seems to be standard policystandard 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.
