I don't know! It's true that WDW didn't leave any little cards in the room explaining that intention (like I've seen at other hotels both here and abroad). Our last two stays were at Caribbean Beach Resort and the Villas at Wilderness Lodge. At both places, we did what I've come to assume is a universally-understood practice: we hung our used towels (that we were okay with keeping) back up, and threw any truly soiled items that we wanted to have replaced (e.g., a couple of washcloths and face towels) on the floor. At both resorts (granted, we only had one trash-and-towel visit at VWL, so maybe we just got lucky there) housekeeping seemed to understand our intent, replacing only the items we put on the floor.
Perhaps at the Grand Floridian, they feel they're giving you a more posh experience with daily fresh towels (or maybe housekeeping just isn't uniformly trained on what to replace and when).