Partly it's a question of cultural differences...
(Remember the old story about how it's impolite in some countries to show the bottoms of your shoes?)
In part, it's considered a matter of respect. In part, it's a health issue. And last, but certainly not least, in part it's the overreaction of a city which is absolutely shellshocked by tourism:
DO NOT do the following!
...Swim in canals.
...Wade in canals.
...Otherwise get too close to canals.
...Sit down on the ground.
...Lay down on the ground.
...Otherwise get too close to the ground.
...Take off your shirt.
...Take off your shoes.
...Wear a bathing suit.
And, never, ever, under any circumstances,
put your feet up on anything!!