Why is it impossible to raze conquered city ? I don't have the option salting the earth when i conquer a city. Do i miss something ?

Before you get salting the earth you need 100% approval rating (check the reasons your new city is rebelling - one of them should be sth. like 'just conquered'). You'll see a percentage indicator on the right side of the city label/sign increasing every round. If that hits 100 it vanishes and then or very soon thereafter you should get salting the earth option. If that's not the case, a) you are playing a faction I'm not familiar with or b) they changed the mechanics or c) it's a bug

Yup, you need to assimilate the city before being able to raze it.