Many people (I suspect the vast majority) would not reuse the iMacs as displays.
Would the total amount of extra hardware inside discarded iMacs (those not used as displays) be less than the amount of hardware saved by reuse of the others?
Would it affect apples bottom line if they couldn't prevent people from reusing their displays? You bet.
Hardware that costs a few cents at Apple's volumes and adds about 1mm of thickness and 2g of weight.
Also, it's standard hardware
They could resell them as displays, given the resale value of Apple devices that might not be unpopular. If Apple actually cared they could easily add the hardware necessary. Would it cost them a few bucks more? Sure, but that's what choosing environment over maximum profit means.
Or they could at least make it easy to modify so tinkerers can quickly turn it into a display without having to destroy the case or something.
Apple is also the king of integrated batteries. First with phones, then with laptops. I'm still baffled they got away with this. Such mindless waste at an incredible scale.
Being extremely environmentally conscientious while designing the packaging isn't going to offset that.