I can only guess that the powers that be decided that they wanted that extra sleeper revenue, so they dis-allowed using the coupons for that.Also: why would those passes not work on sleepers? The price of the sleeper is the same whether it one or two people in them...
Remember that Amtrak loses a lot more in some sense with the sleeper vs. the coach seat. Yes, in theory the cost of the meals are included in the price of the room, but still would be getting a free seat in the room and free meals if they allowed it. One doesn't get a free meal in coach or BC.
And while the coupon does work on Acela, if you want FC on Acela, then both people must pay the FC upcharge. You only get the free BC seat or free railfare.