"Real world trading (RWT), also called real world item trading (RWIT), or real money trading, is the act of trading anything outside of RuneScape (usually real world money) for RuneScape items or services, or vice versa. "
Source:
http://runescape.wikia.com/wiki/Real_world_trading
Basically what this means is that real world trading is basically considered as trading anything from one game for stuff in another game, and / or from "the real world." You can also consider it like a "third party trade" (Ross just made that up on the spot.) Thus selling / trading stuff from BC for anything outside of BC is not allowed, and follows under the RWT rule. However ofc buying membership / ZG coins from other people with BC currency / items is fine, as your trading something in the game, for something else in the same game.
Ross believes this rule should be enforced considering it is up there as a rule, not a suggestion or a warning that if you get scammed, don't go crying to the staff. Bad on staff if they simply let people freely break a rule.