Hi there,
The definition of homebase is in the flight duty and rest periods regulations of SHGM, the Turkish version of which you can reach from here:
http://web.shgm.gov.tr/doc3/sht6a50.doc
The very first item of definitions and abbreviations ("Tanim ve Kisaltmalar") chapter on the first page is "Ana Us" (Main base). And the Turkish word used for the physical location of a base in that definition is "mahal" which when translated into English means "place, local", and not, as you would assume "airport"!
So, no, your company is not doing something illegal, but they are using the definition very succesfully for a city which spreads over two continents and takes about 3 hrs to get from one end to the other...