I think you'll find that its the people who pay for the advertising subsidise both the free copies and the "paid for" copies.
With most magazines/newspapers, the cover price doesn't go anywhere near the actual production cost.
It's the advertising that brings in the revenue, but if there is no (or little) circulation numbers, then there is no advertising revenue. Hence all the free copies to VH rego owners.
In saying that though, aircraft owners constitute Aviation Traders target market. They are the buyers and sellers of aircraft.
The retail trade is generally just enthusiasts who aren't really in the market for an aeroplane.
Also, a newsagent mate of mine tells me that Aviation Trader is one of the best read publications in his shop. So looks like many others are having a free read.