You're couching your defence of this in free market terms, but the truth is that for customers of this ISP these web sites no longer exist in a free market for Internet services. They have managed to skew the market in their favour by artificially lowering the overall cost of using their services and artificially raising the cost of connecting to competing services. You can't have your free market cake on one end of the argument and eat it on the other. Either you're in favour of free markets at both ends, or you aren't.
Yes there is a market for ISP access and consumers are free to chose another. That's fair. Nobody is arguing this is illegal, it not even necessarily immoral for the people that made the deal. It's just business and companies give away freebies all the time for all sorts of reasons. The problem comes if this becomes standard practice, because at that point you don't have an open market anymore, you have an industry cartel that has stitched up the market to divide it up between themselves. See Adam Smith for why this is bad and why, in order to operate equitably and in the public interest, markets need to be regulated in order to remain free.