The problem under discussion is that an onlooker will think that he is slaughtering a sanctified animal, Kodesh, outside of the Beis ha'Mikdash (which is a severe transgression). However, this animal was not sanctified. The fear, therefore, is that an onlooker will *think* that he sanctified (Makdish) the animal immmediately before the Shechitah. This is possible only for an offering that can be brought voluntarily (because a person cannot sanctify an animal to be brought as an obligatory offering that he is not actually obligated to bring).

D. Zupnik