Hi Lepidus,
Fair question, deserves a fair answer! The major freight companies do have their 'own' airports where they set up their hubs (ie/ in Europe FedEx use Paris,UPS use Cologne,DHL use Leipzig, TNT use Leige etc) but other major airports such as Heathrow will still feature heavily because that is where much of the international traffic comes in with 'belly' freight. There is no big deal using passenger flights for most freight provided it is not classed as dangerous goods. In practice most of the major freight companies use their own specilised aircraft. The general public have little idea how big the air freight industry is but I think it is fair to say that freight is where the big money is for business. To give you an idea, use the internet to look at a company such as FedEx, both the fleet size/types and the financial size of the company. It makes BA look like a small flying club in comparison (no offence to my friends in BA!). The freight industry doesn't require the 'in your face' marketing that passenger airlines need to fill seats, hence out of the public spotlight and (generally) understanding. Hope this helps towards answering your question. Best regards, BK.