I imagine it has to do with mechanical stress on the mounting holes. They're probably only set into the plastic frame, and mounting at a more severe angle might over-stress the holes causing the screws to rip out and the tv possibly falling.
if you have some sort of strap to help support the weight while the tv is stowed, i'm guessing it wouldn't be a problem.
2013 KZ Sportsmen Classic 200, 20 ft TT
2020 RAM 1500, 5.7 4x4, 8 speed