They have to be removable to pop off when the tank blows etc.
They also need to be changeable since there are different weapon loadouts
When painted they sit pretty snugly into the space for them.
If they're at all loose you can use blutack or equivalent under the lip of the drone to hold them in.
I never had too many problems with them falling around and I played tau for 3-4 years
link to some army pics
http://www.flickr.com/photos/bloodclaw_97/sets/72157601861878979/