I guess it really depends on whether the hotel is actually charging guests who arrive before 3 pm, even though their rooms may have already been turned by housekeeping.

I understand that for resort properties and large conference hotels, it may be hard to have a significant portion of rooms turned by housekeeping before the official check-in time, and the properties don't want to give guests an expectation that they will be able to get into their room early. That being said, at many properties, I've seen notices published to the extent of:

"Check-in time is 4pm. Guests arriving early can check their luggage at the bellstand if a room is not ready." (from the Marriott Copley in Boston)

I think this accomplishes the message the property wants to get across while being far more customer friendly than saying that you will be charged 50% of the room rate if you want to, for example, get into your room 1 hour before check-in time.