Possible reasons for the hatch to be placed on the seats:

. Damage to a slide
. Creating a danger as pax jump out the window
. The average pax takes longer to get the door tossed than placing on the seats.

Possible reasons for ignoring instructions

. Think the person who came up with the instructions is an idiot
. Despite never having performed the operation, know what is best

Sorry, but I think following the instructions is correct here.

BTW, On the ease of tossing it, this can be tricky. Look at the door as pictured. You are holding with two hand at the top and bottom. If you try to naturally toss it the you will be doing so by pushing the long edge first. And this will not easily fit. You likely need to push it with the short edge first and that requires one hand way in front of the other which not natural.