Why,in particular, have special walls been erected in Paris, for reasons of hygiene?

Well those French guys will urinate against anything that isn't moving, as we all know.

So it serves them right if they get pissed back on:

The Telegraph
Officials said that another prototype on trial was a sloping granite "parapet" around parks that sprayed urine back on to culprits and was difficult to stand on, "particularly after the third beer".