I know what you are getting at, just like the free warranty isn't free, the 30-day no-hassle return policy isn't free, etc., but...the marginal shipping is in fact free.
If I buy two 12" woofers that come out to more than $100 then I get "free shipping". Sure, I can consider the shipping baked into the price. Now, if I buy four of those 12" woofers, the shipping hasn't changed. Thus, the additional shipping was in fact "free". I have probably only placed two orders in the last 10 years that were under $100. If I am at $90 it will probably cost be $10 in shipping for a total of $100. So, I throw in $10 of crossover components and the total is...$100. I literally received the crossover components for free.