That's where services like HotelTonight make the properties very happy - they fill the rooms at deep discounts but HotelTonight takes the "brand hit", not the hotel itself. The Four Seasons can plausibly say "Well sir we didn't give you the $75 rate - that other service did" even if they full well approved it :)
I'm not sure how these things end up playing out (and if HotelTonight shows data that maps how often selected hotels actually have rooms available, that might undercut the hotels' stance a bit), but I know I wouldn't want to rely on it; even planning in advance, in my experience it's not terribly rare that hotels are booked up if you don't call early enough (and it's an in-demand area).
Edit: ah, yup -- that's HT's pitch to the hotels. From the section for "hotel partners": The key to targeting these unique and valuable guests is the IMPULSE DEAL, a rate so you good you only use it when you absolutely need to move rooms at the last second. Since you don’t have that need everyday, our display changes everyday, meaning guests can never predict when your hotel will be offered.
Think about it a little more. Electricity & heat will mostly be on anyway. Complementaries are bought in massive bulk and are almost worthless. They'd have the same number of staff on if the hotel was 80% or 100% anyway. Depending on the size of the hotel maybe you might have to hire one more minimum wage worker.
Hiring the room for a fairly nominal amount would cover all those costs. Upsell them one product, such as a meal, and you've got profit.
You have to remember the majority of the costs are fixed when calculating the cost per guest in a hotel.