Why the Salt is not to be theft. ?

It is said the Salt should not be trampled upon out feet.  Also  the Salt should

 not be borrowed from others and should not give that to others with our hands.

The Salt is the symbolic for the Planet Saturn.  In olden days the Salt was not

 available easily. By making hard trails the people used to accquire that. 

And also it was said with difficulty the salt which was accquired, 

the people used to feel that it was the Amsa (part) of the Saturn and

 the symbolic of the Yama Dharma Raja.

As it was in the custom of people, the Salt was not  borrowed from others.

And also it was not theft. 

At certain age,   it is good that we should not use the Salt in the butter milk or curd.