Apparition & Fidelius

[Arya]

No: HPFGUIDX 90933

Yet another question that ties into my massive and pervasive curiosity 
concerning the Fidelius Charm....

Why do Order members arrive at HQ via the front door and even ring the door 
bell, waiting for someone to let them in when Fred and George show us that 
Apparition is possible within the house?  Is a just something that allows them 
to Disapparate and Apparate while remaining within the house?  If they can 
do this, why do Order members not simply pop in and out without risking 
being seen outside as they enter or take the time to wait for someone to 
answer the door?  

Could this perhaps be linked to the Fidelius Charm?  Say, because F&G are 
already in the house, they can pop and crack as they please and probably 
even leave its confines.  But perhaps Apparition *into* the house from 
someplace outside of the house is forbidden or maybe even dependent upon 
the Secret Keeper sharing the Apparition coordinates or even just plain 
impossible?  There must be something that makes it impossible for the Order 
members to not pop in and out--it's far and away a better way to travel and 
less risky than arriving by the front door.  But what allows F&G to do it in the 
house then?  Ideas??

Arya