It makes sense in a way as the sponsor is supposed to support them so they don't use social assistance because if they do, the sponsor must pay the government back. However, if the sponsor is on social assistance, he or she is supporting the applicant with the social assistance. So technically the applicant becomes on social assistance because, at least in the states, the amount received is dependent upon how many people in your household so the allotment increases accordingly. That could be how the officer sees the situation as well.