It is very safe to take kayam churan more than throwing red chilli powder on someone . But u have completely avoided paras throwing R.C.P. on them but are deliberately highlighting about kayam churna.
Also asim was reluctant to go to low degree to torture them so he being a gentleman thought of something safe . U should appreciate him for this rather than criticizing. Older people generally take kayam churna everyday for constipation . And aarti also said it was just half a cup so quantity was really low of the powder used. Asim did it so reluctantly whereas in the first task of the show where he had to hold hand with abu ji he had to go through lot of torture.