It's the new business model of revenue generation, just like in "free-to-play" games. If a site has free registration and no "requirement" to tip, then why do people even use the term "freeloader"? People download and play free-to-play games without ever spending a penny on any given game, but in any given free-to-play game, the other players don't have a clue who supports the developers, and who does not. But, the game developers sure do know. Can you imagine what would happen if a game developer "outed" players for being "freeloaders" to other players? At the end of the day, all the game developer can do is hope that the "conversion ratio" of free players to "paying players" is enough to keep the game afloat. In F2P games, that conversion factor is statistically between 3-5 percent, and of that 3-5 percent, less than .1 percent are whales, but the important thing here is that the whales help keep the developer afloat.
It's no different with adult sites who employ free registration. They are counting on the fact that a percentage of users will convert from free users to paying members, and a certain percentage of those converted members will be considered "whales". I don't know the conversion stats on adult sites, but it has to be good enough for sites like MFC and CB to stay afloat for as long as they have. I guess it's a pet peeve of mine whenever I hear someone use the term "freeloader" when talking about users on sites that employ this type of business model. But, I think most successful models on these sites have figured out it's up to them to build a following of regulars. Most of them know, or should know, that the site is just the venue.