Why is that so? as in what is the point of being supported, but not being able to support in that initial 48 hours?
Isn't that obvious?
If a person joins a tribe, the tribe are more likely to give him res so he can climb up through the ranks. if a rim tribe of course.
If a person is well established you don't want him to send support within those two days to the rest of the tribe if the tribe is good at poaching good players.
For example: My tribe just poached a 5k player which doubled are tribal score today, so if he could send little me res now it be unfair on the bloke beside me.
I don't know if i explained that in a good way but it makes sense. i can see why the devs did it.