The point is that authorized service shops are provided with the training to perform repairs on these things, so whether the display/glass is glued to the body or not, people are going to be able to take their machines to a shop, have it repaired, and receive it back in working condition. The iMac, if we're following the analogy in an honest way, doesn't have its hood welded shut. (Because iMac mechanics can, in fact, repair them, even if it is more difficult.)
Further, as far as the user is concerned, breaking out a heat gun is just as inaccessible as buying a Torx screwdriver (they don't own one, I promise) and suction cups to pull the glass and display off the old model. They're going to a repair shop or an Apple store 99% of the time.