Why can't you plant in the rightmost 2 or 3 columns of the lawn during Zomboss battle levels?

You can't plant on the 3 most far right columns because they are occupied by Dr Zomboss. Plants cannot exist where he stands.

In the original Dr Zomboss fight in Plants vs Zombies, you could plant on the 4th most far right column, which is the flat section of the roof where Dr Zomboss stands. But when Dr Zomboss is standing, he will crush any plants on that column. This makes me think that if the game did let you plant where Dr Zomboss was in Plants vs Zombies 2, the plant would automatically be crushed - the game is preventing you from wasting a plant.