Because they inevitably take the only shady spot.

Those who insist on eating outside at a restaurant should have to take the seat in the sun.
If they wanted to be out in the warm sunshine, they should get the warm sunshine.
They shouldn't make someone else take it.
Because eating outside was their choice.
And eating outside means eating in the sun, not making someone else take that seat.
Otherwise, they won't be getting the full eating outside experience.
And if they wanted shade, they should have eaten indoors, where it's nothing but shade.
Except sometimes near the window, where there are usually shades.