When asking for an early arrival, the result may be contingent upon who you ask. Any requests prior to arrival will be sent to the Reservations Department. The people working in Reservations are trained to get the most revenue possible for every room. Lots of Reservations/Revenue Managers receive bonuses if they exceed monthly budgets. Those people will typically push for you to secure the room for the night prior to arrival. Alternatively, they might have programs with early arrivals fees as noted in the OP.

Once you arrival at the hotel, you're dealing with the Front Desk. Most Front Desk personnel are paid hourly, which usually means that any additional revenue isn't going to pad their paycheck. Unless there is specific training about how to handle early arrivals, many Front Desk staff will try to check you in. It makes you happy, which makes their jobs easier.