Wonka said before opening the door to the main room that you can eat “almost anything” in the room. Wonka never said “except the chocolate river; that’s off limits.” Augustus is in a room where people are eating teacups, people are eating giant mushrooms, etc. And he takes a drink from a chocolate river. It’s totally in line with the vibe of the room to drink from that river. How could he know not to do this unless told not to?!?