I guess I see what you mean
they could still stick the top cover down with an inner cover if they just come up thru the opening, but it creates a barrier for them.
They see the inner cover as the top of their cavity and seal the bottom of the inner cover to the top of the super. They do not consider the space above the inner cover as "habitable" and therfore have no instinct to seal the telescopic cover to the inner cover.
You dont want to use an inner cover under a feeder because it limits access to the feeder. Yes they will seal the bottom of the feeder to the top of the super, but just like an inner cover and unlike a telescopic cover, you can easily shove in a hive tool to seperate them.
Using an inverted glass jar over the inner cover hole with an empty super is a better feeder alternative than the commercial feeders. First of all, they are free, they provide quicker/better access the the bees (bees can feed regardless of the temperature) and they are dishwasher safe.
Quart jars fit into medium supers and gallons fit into deep supers.click image for larger view