Why do they do it?
Cause it's a requirement at ports without VHF on the ground so has been standardized by some pilots to do it all...
If you make a call on hf you are required to make the same call on area as well... These guys would of been doing this prior to the change to class d last November and it's stuck now. The line between sop's and not eventually gets blurred if they all do it and forget which places it is actually required to do so...
I could also see some guys argueing the point that if it assists in the safety of their aircraft then stuff the rules they're doing it regardless... Which is fine IMO