It would be nice if they removed the cost of the food from the sleepers, so the sleepers are cheaper, and the sleeper pax pay for their food. I'd suppose some passengers just want a bed to sleep in at night, and they don't need to eat in the diner car.
Amtrak has a finite supply of sleeper rooms, so from a revenue perspective it makes sense to price them at the highest point the market will bear. That premium price includes meals which, as already noted, are also tied up with the dining car budget.
Amidst all the talk of a less expensive coach configuration, I have to wonder about the potential of a premium-coach option with meals and other amenities included similar to sleepers which would, again, benefit the often attacked economics of the diner.