Possible Flint: Fireplace in the common rooms

[artcase]

No: HPFGUIDX 83375

How/why do students receive mail by owls? 

If their parents could use the floo network to communicate (much like 
a telephone/transporter) why would they resort to the much slower, 
less efficient method of owl post?

AND for that matter, how do the teachers keep the student from 
leaving the school/parents from coming into the school via floo?

My only guess is that it is restricted in some manner, which makes 
Sirus's useage ("broke into a wizarding house...")highly unlikely.

Maybe I'm just reading  too much into it, but what is stopping LV 
from using a wizarding house (in the same manner) and dropping in on 
Harry some late night?

Comments?